Gears of War 1s single-player was average, the multi-player was absolutely fantastic. Overall, I'd give it a 9/10.
Gears of War 2s single-player was slightly above average, the multi-player was absolutely atrocious. Overall, I'd give it a firm 7/10.
Gears of War 3 needs a good story; none of that MARIIIIIIIIAAA crap please. And of course, it needs some good multi-player.
Because when your single-player campaign is meh, and the multi-player is bleh, there's just no replay value.