Not a game for the weak of stomach.
Several additions come to mind here.
The new sprite system, 32bit source art of course, entirely new AI system, guorad shaded models, localized damage zones and skins. We've more than doubled the original code that makes up the Quake2 engine. There is a definate advantage in spending more time tweaking and working on gameplay, than enhancing an engine beyond the competition. Depends on the card. Most cards have very good quality 16bit performance, like the TNT, Rage128, G200, etc.