While considered to be a gore mod, it goes further and alters the gameplay by changing many other aspects, such as the sounds, graphics, and combat.
One such feature is the increased difficulty, making enemy behavior more unpredictable and dangerous (many attacks do double the normal damage to the player) and altering their attacks. It makes the animations smoother and gives the player new abilities.
Perhaps the most obvious and eye-catching feature of Brutal Doom is the adding of features like additional blood (blood gets splattered on walls and ceilings if enemies or the player get hit), the ability to blow off body parts with strong weapons like the shotgun, and the addition of special iluminating effects on projectiles and pick ups. It is compatible with ZDoom, GZDoom, Skulltag, and Zandronum.
- Dramatic increase of gibs creating a much gorier atmosphere.
- 3D bloodspots allow all textures to be covered with blood, including the floor and ceiling.
- A more dynamic interface. For example, blood splashes on the screen if the player kills an enemy up close, and glass cracks and/or bullet holes may appear on the screen when the player gets injured.
- When the player is critically injured (30% health or lower), the screen gets stained with tiny blood splatters, and a dripping sound is played intermittently as the marine's blood drips continuously on the ground (albeit this is just a visual effect which adds to gameplay realism and does not incur in further health loss).
- New death animations for all enemies. Body parts can be torn apart by regular fire on various locations on the enemy's body, cut in half with the chainsaw, melted with the plasma gun, burnt into smoky scorch by fire or electricity-based attacks, and blown to smithereens by explosives.
- Corpses can be further mutilated upon death.
- Many enemies have new attacks (more details below), and many enemy attacks do up to twice the normal damage.
- Many of the game's weapons have been altered or tweaked in various ways, such as by making it necessary to reload or by adding secondary functions (see also below).
- The berserk pack's functionality has been expanded. With it, the player can perform fatality-style finishers on monsters, as well as grab Lost Souls and explosive barrels and throw them. Performing fatalities on monsters replenish player's health - the more powerful and dangerous the slain enemy, the more health will be recovered. The player is completely invulnerable while performing a fatality, no matter if surrounded by monsters or taking damage from them. Also using the berserk pack, the player can pick up stunned zombies and imps (a "stunned" monster will have had one of its limbs blown off and be kneeling on the ground) to use as meat shields against projectiles. While using a monster as meat shield, the player is limited to firing only the assault rifle and becomes vulnerable to monster attacks while firing the weapon. A meat shield can also be thrown towards other enemies, while "intact" or after being destroyed, and cause enormous damage by pressing the alternative fire key, but this action will be automatic whenever the player needs to reload the rifle.
- Implementation of headshots for all monsters - except for the Cacodemon, Lost Soul and Pain Elemental. Headshots deal as much as double damage (depending on the weapon), and can gib the monster's head for one of many special death animations (see the screenshot at the right). Autoaim does not assist in getting headshots.
- Invisibility spheres are replaced with captured marines. If the player punches or kicks the marine, he will be freed and will gladly help the player fight against the demons. The captured marine holds random weaponry, with multiple playthroughs of a level resulting in differing weapons that the marine will fight with (more powerful weapons have smaller probability of occurring in the marine's hands). The marine can punch enemies at close range and even perform fatalities on them (being fully invulnerable in the process) to replenish his own health, just like the player can do. The demons around the captured marine will attempt to kill him before the player can free him. If the marine is killed in battle after being freed, his weapon will be left for the player.
- If the player kills certain monsters (or shreds their corpses) with the chainsaw or rocket launcher, they can steal their weapons - for example, the Revenant's hellish rocket launchers and the Mancubus's flame cannon, both mapped to key 8 (key 9 for the Mancubus cannon in version 0.18 onwards).
- Enhanced lighting and shadow effects for decorations and projectiles similar to those found in the Doomsday engine.
- Enemy bullets now act as projectiles, albeit very fast ones, allowing the player to dodge them given sufficient time and distance.
- The fists are faster and can make quick jabs. They are also a silent attack and will not alert enemies. The alt-fire throws a slower but more powerful right hook punch. Punching a Zombieman, Shotgun Guy, Heavy Weapon Dude or Imp from behind before the enemy has noticed will snap their neck, killing it instantly without alerting other enemies.
- A kick attack has been added to shove enemies away from the player, similar to the one in Duke Nukem 3D. It can be bound to a specific key. When performed in mid-air, the player performs a flying kick with his foot turned 45 degrees left-upwards, which deals much more damage.
- The chainsaw uses the Doom 3 Chainsaw sound effects and blood splatters on it when sawing an enemy.
- The useless pistol is replaced by an assault rifle, matching the player's sprite, which unlike the original pistol remains useful even late in the game. The assault rifle has a decent fire rate (close to the original chaingun) and is one of the most accurate weapons in the mod. The alt-fire mode allows the player to aim down the sights to snipe enemies (starting from version 0.18, the aim is made via a realistic telescopic crosshair embedded on the weapon). It has a clip size of 30.
- The shotgun now has a black synthetic pump and stock instead of wooden ones, and also has a vented heatshield on the barrel. Like the assault rifle, the sights can be used for more accurate shots. It now fires 10 pellets per shot to match the super shotgun's 20. It has a magazine size of 8.
- The super shotgun has a faster reload time, and at point-blank range will deal an additional damage boost. It now has the option of firing only one barrel at a time, thus shooting only 10 pellets.
- The chaingun is now correctly referred to as a Minigun (as well as resembling the more traditional design of said guns), and fires 50% faster but also requires a spin-up and cool-off before and after firing respectively, similar to the plasma gun. The alt-fire mode makes the barrels continuously spin, further increasing the firing rate but also drastically decreasing the accuracy and alerting monsters. The recoil from continuous firing will constantly push the player's aim upwards (version 0.18 only). Unlike most of the reworked guns, it does not need to be reloaded.
- The rocket launcher has a much smaller splash damage area, but has a faster fire rate and causes 50% more damage on a direct hit. It holds 6 rockets per clip. It is also the only weapon that reloads automatically after emptying a full ammo clip or when the players selects it for the first time.
- The plasma gun now deals roughly double damage and can cause minor splash damage to enemies (and players, if the bolt hits a nearby wall), but fires at a slower rate. The alt-fire mode fires a spread of plasma bolts in a shotgun-like spread, which can do as much damage as two rockets. It has a clip size of 50.
- The BFG9000 uses the Doom 3 BFG sound effects and the primary energy ball deals double damage. Instead of the original BFG's hitscan tracer attack, it simply deals damage in a very large radius. It can also fatally injure the player if the energy ball explodes too close (only applies if it hits an inanimate object or obstacle and not an enemy), again unlike the original BFG. This revamped version of the BFG9000 now expends 50 plasma charges per shot.
- Most weapons cause kickback, especially the Super Shotgun and the BFG9000 - the bigger its firepower, the farther backwards the weapon pushes the player.
- In versions prior to 0.18, firing weapons too close to walls and other solid surfaces dealt minor shrapnel damage to the player.
- The player is able to flip monsters off by a bindable command known as "Offend". This command will alert nearby monsters as if the player had just fired a weapon. There are other bindable commands, such as "Wave". Pressing the wave button during a fatality (see below) will allow the player to see his own wave animation in third-person.
- There are several voice acted taunts and one-liners that can be spoken with keys J and K. Many of these quotes are taken straight from the infamous Doom comic.
- The player's ammo capacity for bullet weapons has been increased from 200 to 300 (and from 400 to 600 with a backpack).
- Both the Grenade Launcher and the Rail gun from the Skulltag source port have been given new sprites and attributes. The Grenade Launcher is a standard issue M79; it has a much larger blast radius than the Rocket Launcher, but fires much slower. The Rail gun fires at a slower rate than the vanilla Skulltag Rail gun but uses half as much ammunition, has a scope for zooming in, and reloads after 5 shots instead of 4.
- Sprite change of rockets and box of rockets, backpack, armor bonus, armor and megaarmor.
- Zombiemen will sometimes rapidly fire off 5 shots without pausing. Also, both he and the Former Sergeant must stop to reload their weapons after a while.
- Shotgun Guys now fire 8 pellets per shot instead of only 3.
- Wolfenstein SS officers changed radically into soldiers in black Nazi-style clothing with demonic/undead appearance and fanatical military voices, and possess sharp AI that makes them extremely deadly enemies, especially in groups. Their powerful MP40 submachineguns, mapped to key 8 (key 0 in v0.18), can be picked up and used by the player. The MP40 has a higher rate of fire than the assault rifle and a 32-round magazine.
- The Imp now has a jumping pounce attack like its Doom 3 version that it will use at close range, and can sometimes shoot two fireballs in quick succession. Its projectiles are now much more realistic, with several sound and visual effects. Whenever the Imp successfully slashes the player, the red scratch graphic marking used in Doom 3 is shown on the screen, its position on the screen vaguely hinting where the slash came from.
- A dodge system was introduced in version 0.18, allowing some enemies (mostly the weaker ones, such as Zombies, Imps and Lost Souls) to occasionally dash sideways to avoid projectile attacks.
- Demons, by nature of being a melee-only monster, by default will always perform a fatality on the player, depicted by the monster chewing on a few gibs or the killed player's head. A Demon can also burn to death if struck by a player-thrown Lost Soul or any fire-based attack: when this happens, the burning-to-death Demon will run aimlessly at about half speed, repeatedly crying in pain while its body burns incessantly. This will continue for a few short seconds until the Demon collapses dead as its burning corpse turns black, and the flames extinguish shortly after.
- Spectres are completely invisible - save for their eyes, making them much harder to detect - however, they briefly flash into a translucent state upon attacking and taking damage. When a Spectre is killed without being gibbed, its corpse loses the translucency power (in the form of a red aura which "smokes" upwards from it) and becomes permanently visible.
- The Cacodemon can sometimes shoot two consecutive projectiles and shunt itself out of the way of incoming attacks. It also floats a bit faster than its classic Doom counterpart. Aside from being given greater speed and agility, another difference is the Cacodemon's melee biting attack being now fully audible with a chewing/slashing sound effect, unlike the classic Doom Cacodemon's silent gaping maw. If a Cacodemon bites a player to death, a sideways animation will be shown depicting the monster eating the player's upper torso then dropping the player's mutilated lower half to the floor as the monster squares back up to the camera.
- Melee-oriented monsters, such as Hell Knight and the Demon, will perform fatalities on the player and even other monsters if they kill them with a melee attack.
- The Hell Knight now can sometimes shoot three consecutive projectiles, in a similar fashion to the Imp and Cacodemon. However, when the Hell Knight performs the triple fireball throw, the animation for this depicts the monster throwing right-handed, left-handed and right-handed in that order. A copy-and-paste job of his right-handed throw is used essentially to mirror the throwing motion when a Hell Knight alternates throwing hands. Both the Hell Knight and Baron of Hell throw right-handed by "default".
- The Baron of Hell has gained two new projectile attacks: in the first one, it launches 3 projectiles in a wide spread instead of one. The animation for this tri-spread attack is shown by the Baron clapping his hands together instead of his usual sidearm throw - this animation is essentially a cut-and-paste job of his sidearm throw animation horizontally duplicated to look like the Baron swings both arms in order to clap his hands together and launch the attack; in the second one, albeit quite rare to happen, the Baron can grab and throw a dead monster's corpse (usually a Zombieman or Shotgun Guy) like a missile towards the player, in the same concept of meat shield described above, incredibly causing massive damage - but this attack cannot be performed with corpses that were already placed in the ambiance by the map designer.
- Chaingunners, Hell Knights and Barons of Hell can be kicked in the crotch, stunning them for a few seconds (applies only from version 0.18 onwards).
- Arachnotrons are much larger, around the size of a Baron of Hell, and also move faster. Their plasma shots now are the same blue projectiles fired by the player's own plasma gun.
- Lost Souls now deal less damage, but fly around and attack much faster, which can potentially cause unlucky or slow players to miss attacks constantly - making them one of the more frustrating enemies in the game. Their health is also halved compared to their vanilla version. Their flames have been removed from the sprite and are now generated by particle emmiters. Upon death, Lost Souls explode in a fire puff and leave small bony chunks on the ground.
- Pain Elementals explode upon death, causing heavy damage to nearby monsters and the player as well. Said explosion can be prevented only by killing them with an execution/fatality.
- Revenants always fire two guided missiles at a time (matching their animation of both shoulder cannons lighting up when firing), and non-guided missiles are fired in volleys of four. Guided missiles have a much more limited fly pattern (they tend to travel downwards to the ground unless they can reach their target or hit any obstacle) and are now indicated by a reddish glowing color (non-guided missiles have a yellowish lens flare effect).
- Mancubus fireballs have a small radius blast area that deals slight fire damage. They leave damaging flames for several seconds after hitting a surface.
- Arch-Viles are less powerful, as their attack takes longer time to charge and they have less hit points, however they can resurrect enemies without having to stop and face the monster's corpse. Their appearance also changes, as they now sport large stereotypical ram-like demonic horns.
- The Spider Mastermind's minigun is now a high-calibre gatling gun loaded with explosive rounds (each round, although much weaker than a Cyberdemon's rocket, is still very powerful). Its reaction time is much quicker, almost instantaneous, giving the player little time to get out of its way. However, its bullets travel slower than normal bullets, so they can still be avoided if the player has a lot of room to circle strafe. Perhaps the most dramatic change to this monster is that it has four times as much health as its vanilla counterpart (3000 to 12000), although it will take increased damage from some weapons. The creature's mechanical chassis absorbs most of the damage inflicted (it nullifies around 7/8th of the attack's original damage value), producing metal sparks from gunfire and chainsaw attacks. Its death animation is also enhanced, and the resulting explosion can badly hurt or kill players or monsters too close to the blast (only applies in versions older than 0.17).
- The Cyberdemon has a new melee attack: a devastating stomp, which against the player almost always means instant death. In addition, it shoots its fire-engulfed rockets faster and for longer (four rockets instead of three) in a wider angle, giving it a limited ability to lead its shots against circle strafing players. Like the Spiderdemon, its death animation is visually enhanced to deal massive damage to anything near the explosion (again, only below v0.17). As an aesthetic correction, the rockets are now fired from the Cyberdemon's rocket launcher arm instead of the middle of his legs, akin to his Doom 64 version. Starting from version 0.18, using the "Offend" command to flip the Cyberdemon off will enrage him, causing him to fire his rockets much more aggressively - he now fires six consecutive rockets, in a much faster rate and having smaller interval time between walking around and firing again.
- Decorative props, such as trees, hanging corpses and lights, can be destroyed. Destroying hanging corpses will occasionally reward the player with health and armor bonuses.
- The player and monsters now "wade" through a liquid instead of walking on it (their sprite appears partially submerged), and wading through liquids causes a splashing sound to be played. The various liquid textures are also enhanced.
- Enemies randomly drop demon strength runes upon death, which act similarly to berserk packs (albeit they do not recover health as much as the berserk pack does).
- In certain versions, Commander Keen's head is replaced with that of famous pop star Justin Beiber.
- Brutal Doom also has certain mutators that alter the main gameplay of this mod. One mutator even replaced the Assault Rifle with the useless pistol with changed graphics.