It looks to me like Gears of War 2 is taking a very Gears of War 1.5 approach, which isn't necessarily a bad thing if you were a big Gears fan. But "horde mode" is the thing that has me really looking forward to the game. From Kotaku's write-up:

The idea is to survive wave after wave of Locust hordes that the game throws at you. You lose if every member on your team is killed. You win when you kill every Locust in a wave of the horde – after that, the game pauses to show you which players had the most kills and then chucks you back into the map with another wave.

It's a pretty basic concept, and well established in many kinds of game. In fact, Gears developer Epic patched it into Unreal Tournament 2004 a few years ago. But it's a great way to play co-op multiplayer, or even single player. Just throw endless waves at me and let me see how far I can get!

Kotaku has also posted some observations about the opening of the campaign, which sounds like a bunch of Gears of War 1 all over again.