It's the weapons really. Unlike other shooters, the weapons here actually work very much differently from each other. In a game like COD4, the guns pretty much divide themselves according to their operating distance (sniper, long distance; machine gun, short-mid distance) and they just about work the same. I guess I get drawn by variety.
Well if you liked Quake 3 (and you're an idiot if you didn't), you'll love R2. The guns are really unique and its fun to experiment on the effects.
I just got a mic today (for $5 YAY) and the 60 player matches can either be utter chaos (with a random team) or a very cohesive death brigade (with a premade team). The battles are truly epic with so many bullets of different types flying everywhere; the carnage is dramatic!