The DN3D-style quick boot is great for shoving back monsters, reloading and secondary fire make firefights more interesting without bogging things down, the sniper scope is a surprisingly excellent addition, invaluable against the hyper-aggressive firearm enemies, and best of all: the monsters are much more aggressive, several with new attacks and movement patterns (the Imps' unpredictable leaping completely revitalises a very, very tired enemy). You can't just waltz around them comfortably now, especially as the bigger ones have instant death attacks up close. Getting cornered by a large monster in a corridor = pants-shitting back to the wall terror! They even bust 'em out on other monsters!

Pictured: A Baron tears a Pinkie a brand new asshole starting from the roof of his mouth, as Doomguy looks on in manful silence.
The gore isn't a total gimmick either, since murderizing enemies with the Berserk pack (or stomping on dying ones) nets you health bonuses. The Berserk kills are handled with some slickness. Clobber a monster with a right hook (or several quick lefts) and BOOM instant death grapple in third person, while you're free to target the next chump. It looked like a stupid gimmick to me at first, but it's pretty fun (and practical) to tear through crowds of low-to-mid-level enemies this way. Bigger ones can really ruin your shit if you're not careful about wearing them down first.
You can even rescue captured marines who'll support you until meeting horrible deaths (saw one poor guy get eaten whole by a Caco, shit was like the end of Jaws only backwards
