Gears 2 expandes on the experience in many ways
The way I see it, there are four things that really improve Gears 2 over the original.
One of the biggest complaints about the original was that campaighn was short and the gameplay was not varied enough. Although Gears 2 seems short it is many times more enjoyable than the original and offers many varied elements in gameplay.
The addition of Horde Mode is genius. Lets face it competitive online with 10 year olds yelling in your ear that you suck and taunt your dead body gets rather boring. Horde mode offers a competitive online challenge but in a cooperative manner....its really fun fighting wave after wave and climbing up the leaderboards.
Bots in multiplayer....the inclusion of bots does two things. First it eliminates lopesided matches and lastly since the AI is very good....its as much of a challenge if not more so than public multiplayer with the added bonus of not having jerks yelling in your ear.
Lastly, the improvement in graphics is very noticable. If you don't think so, play Gears 2 for a bit then pop in the original and argue again its not noticable. There is alot more color, the textures are improved, lighting and shaders are far more advanced....all this makes for a superior visual experience over the original.
Gears 2 is one of the best if not the best shooter experiences on X360. Sure, COD4, Halo 3, or even COD: WaW maybe more popular on Live, but all those are first person shooters, and in my opinion, not as intimate of a experience as given as a game with a third person point of view.