Prey is INNOVATIVE because of three totally new FPS features--Portals, Spirit Walking, and Rotating Gravity. Buy it now!
I will admit that Portals are more eye candy than anything else offering only a limited enhancement to gameplay (imagine shooting through a unidirectional portal at an enemy who doesn't even realise there's a portal beside them!!!). But Spirit Walking and Rotating Gravity are extremely innovative!!! Play deathmatch for half an hour and you'll see how different it is from everything else out there. It's so much fun standing on a wall just above a door shooting at people as they enter, to see them totally flip out not knowing where it's coming from!!! :) Or running around a level flipping from wall to wall to wall. Although a little nauseating at first, you start felling like a fighter pilot after 10 minutes.
Prey also looks brilliant. The Doom 3 engine is still 1st class and is definitely the best engine at rendering interiors scenes. Simply compare the difference is detail between the Citadel levels in HL2 and the detailed organic look of the levels in Prey. Two features that are a bit dated are the lack of support for HDR and the puppet like facial animation system, although these limitations do not retract from the gameplay experience.
Over all I think this game is one of the top releases of the summer and is compulsory buying for any FPS fan.