New death animations and fatalities, complementing introduced monster variants.
Larger variety of Doomguy's voice commands: taunts, as well as one-liners (which can also happen automatically, if the player slays many monsters within a short period in a particularly brutal fashion) and weapon acquisiton quotes.
Additional player abilities, e.g.: a "dash" dodging maneuver (performed by tapping twice a movement key), double jump, sliding, or taking meatshields (by punching a fatally wounded Zombieman/Sergeant/Imp while berserk).
Extra power-up types and inventory items.
Expanded monster roster, with most of the monster types having at least three other "variants" - alternate versions, of increasing threat level and various abilites.
Few, selected weapons have "weapon upgrades" tied to them, which are their improved versions that replace the old weapons (but can be obtained without the base weapon in possession) and can be found only on Tier IV. Most of the weapons include tertiary functions (also called "Weapon Specials").
Expanded weapon arsenal, featuring over twice as many weapons than in Brutal Doom (v20), and some of them inspired by Quake and Duke Nukem games.
Tier IV: levels 21-30 (and secret ones) By default, the spawn progression follows Doom 2's "episodes", and accordingly, it is divided into four "Tiers": The exact active actors can be switched in the mod's options.
Dynamic spawn system, affecting what monsters, weapons and items you will encounter.