B-b-b-but Metriod Prime 1-3...
B-b-b-but Bioshock...
B-b-b-but Serious Sam...
B-b-b-but Condemned...
B-b-b-but Perfect Dark...
I could go on, but I think others have beaten me to the point. (though if I'm the first person to bring up Metriod, I'd be pretty damn depressed)
Actually, I think you're more or less right in that most FPS's follow around Halo's example like it's the freaking LAW and deviating even slightly from what's already come is something nobody seems willing to do unless they're copying Call of Duty, instead.
Fortunatly, I've discovered that you can use this as a way to judge how good a game will be before it's even released; if it looks like a Halo clone, skip it, as you won't be missing out on anything mindblowing, anyway.
This rule has helped me avoid drivel like Area 51, Haze, The Conduit, Turok, Jericho and Wolfenstein, and for that I guess I can be thankful.