Well I'm going to have to agree with Hammer; Halo was one of the best FPS I ever played. The AI in that game was just incredible. I mean of course it did stupid things from time to time; but it was the first game I ever played where my enemies actively attempted to flank me (on Legendary). I have still yet to play an FPS (admittingly I haven't played much since then) that has done that. And it was "realistic" in a sense that the Flood used dumb tactics like bum-rushing you, while the Covenant (being the intelligent race) used cover and flanking/rear manuevers.
And I think that's what made Halo so replayable, at least for me. It didn't need multiple characters, or multiple storylines. The game just had so much action in it with so little dead-space; there was always a firefight to be had. I can pop in the disc and play a level or two of jam-packed good action, over 2 years after the game's been released. Who want's to spend half their time walking around dead space with one or two enemies popping out and attacking?
Also, about the dynamic lighting, Xbox is pretty much the only home console that can handle real-time dynamic lighting. PS2 "fakes" it (as seen in Splinter Cell), meaning areas are pre-ordained in shadows. PS2 can handle it on a small scale though (like a shadow for your character on the ground); but Xbox is the only system that can do large-scale real-time lighting/shadows.
I'm sure Killzone will be a solid game, and it may even turn out to be a fantastic game. But at this point, I'd put my money on Bungie, because like Hammer said, Bungie does have a very good development history. They rarely make sucky games. And after seeing the Halo 2 trailer, I can't wait to get my hands on that game early next year.