Frankie O'Connor at Bungie gave an interview near the beginning of this year, and one of the questions I found particularly interesting, especially because I've only recently had a chance to play Gears of War myself:
XCN: Are you influenced by other games like Gears of War, would you consider implementing some things like the Gears of War cover system in Halo 3?
Frank O'Connor: We're always asked about the influence of other games on Halo and recently the 'other game' often seems to be Gears of War. Will the cover system influence Halo? The honest answer is no. The gameplay for Halo 3 was decided long before Gears of War even shipped. If we put the cover system in Halo it would ruin the game because it's not about cover. It's about big encounters and open spaces and vehicles and so on. Gears of War is about cover. Sticking something like that onto our game wouldn't be a good idea. Anyway, there is a cover system in Halo - it's called ducking behind objects and using the environment to shield you from harm. But we won't be putting a cover button in, and it's certainly not the X button that we're often asked about.
Whenever a new game comes out with an ostensibly "new" feature it seems there's an exchange that runs something like this; where a journalist asks Developer of Last Year's Hit Game if they are going to implement Cool Feature from the newly released This Year's Hot Game in next year's Sequel To Last Year's Hit Game.
Whether the feature is appropriate for that game or not scarcely matters; it's a hip and trendy feature. In the case of Gears, it's the Cover button.