I've never been a big fan of shooters, but what I've seen of Gears of War impresses me greatly.

Halo always bothered me in that you can just run out and shoot things and still have a good chance of coming out ahead. The fact that Gears of War REQUIRES you to make good use of cover is a definite plus in my eyes, and the way the cover system is built into the mechanics is top-notch.