A better story, improved graphics, and fun new modes make Gears of War 2 a worthy sequel.
The Campaign mode is much better this time around. Though the story still isn't great, it is definitely an improvement. New characters are interesting and new enemies are great additions. Be ready for some tugs at the heartstrings when you play, though.
The graphics speak for themselves. Whether it is making your way through an abandoned lab or a sunken city, the visuals are astounding.
The gameplay itself has not changed a whole lot. Veterans will be mowing down Locust quickly and new players will pick up the controls easily.
The multiplayer is still very good. In fact, the only three problems I've seen are long matchmaking times, lack of a straight-up deathmatch, and people who insist on using nothing but the chainsaw (but to each their own). One of the most exciting new features is the Horde mode, which puts you through wave after wave of increasingly difficult enemies (No, you wont fight any Brumaks). It is an immense amount of fun to get with some friends and just mow down Boomers, then scream for help when you start crawling as a last resort. The Horde is also an excellent tool to get many of the games achievements, because of the variety of weapons.
In short, Gears of War 2 is a fantastic game that should be experienced by all action lovers.