I've been asked this question several times. I'm usually unable to properly verbalize it, especially since I don't "game" in general (PC *or* console) and therefore am not really familiar with all the other FPSs out there.

But this guy seems to know:

"Halo's gameplay was amazing in that it seamlessly integrated top-notch first-person shooting with incredibly fun third-person vehicular sequences and outstanding friendly and enemy artificial intelligence. The game's subtle innovations--the tactical consequences of such things as having recharging energy shields, being able to carry only a couple of weapons at a time, the ability to throw powerful grenades in between shots, and the option of dishing out fierce melee attacks--also did a lot to differentiate Halo from other shooters, and proceeded to influence subsequent games. Halo's multiple, well-balanced difficulty settings, two-player cooperative campaign option, and assorted multiplayer modes also ensured that the game had tons of lasting appeal. All these factors contributed to the game's well-deserved success, and they're all back in Halo 2. For the most part, the sequel takes an "if it ain't broke, don't fix it" approach to its gameplay--everything that you loved about Halo's action is back in full effect here. At the same time, the handful of new additions in Halo 2 are well thought out and well implemented, enriching the gameplay and making it seem fresh but still familiar."

from
www.gamespot.com/xbox/acti...eview.html

this article seems to echo Hammie's sentiments:

www.gamepro.com/microsoft/...39489.shtml