What we should all appreciate about Gears 2 is that the devs did not make it Gears 1 with more stuff. Instead of this game having five berserkers, 20 seeders, and a boss that was just Raam 2.0, the people at Epic said "Gears 1 was Gears 1, Gears 2 will be different."
Sure, it's mostly the same game with the cover fighting, but the pacing of this game is 20 times faster than the last. You'll be deep underground one moment, in a worm, in a haunted mansion-like place, up a snowy mountain, and then fighting a water monster back underground in not a lot of time. There is so much variety that some of it seems tossed in, not very hard, and more than a bit unnecessary, but it does help the game fly along.
Gears of War 2 doesn't do anything new, sadly, but it's shooting mechanics, story, and enjoyable gameplay make for a great example of a game that is just fun to play. Not to mention the multiplayer is about as good as it gets.