Everything, and much more!
The Campaign is longer than it was in the first, and definitely more immersive. It has a higher replay value with the added items to collect throughout. Although the story in the campaign brings up more questions than it answers, the campaign itself proves to be an amazing experience. One of the new co-op features is Horde mode. 5 players cooperatively take on wave after wave of increasingly powerful Locust. This truly brings out the game's tactical side.
The Multiplayer utilizes Epic's Unreal Engine 3 to the max by enhancing the physics system greatly. It also uses a matchmaking system which can be slow at times but puts forth an experience much better than the first. With the elimination of host advantage and the balancing of weapons, the multiplayer is one of the greatest aspects of the game. Weapon changes include: addition of a chaingun, mortar, flamethrower, and burst pistol, and changes to the sniper and shotgun. The one shot down previously seen on the sniper has been removed and the unbalanced power form the shotgun has also been removed to further balance the system. Instead of a shotgun-fest seen in the original, Gears 2 utilizes every single weapon on the map. There are also new multiplayer gametype additions to the mix. Wingman is 5 teams of two fighting against each other for the highest kill count. Submission is a version of Capture the Flag in which players must down an armed AI and take him hostage. Then, they must walk the AI to a designated area in order to win. Guardian is a new take on Assassination in which players are allowed to respawn so long as the leader is alive. Of course, the old modes are still there: Warzone, Execution, Annex, and King of the Hill.
The graphics and audio system have seen a drastic improvement as well, from the splash of blood, to raindrops, water movement, the sound of bullets tearing through flesh, and the favorite chainsaw tearing things apart.
Basically, this game will not disappoint anyone who is not afraid to adapt to changes made and try a new method of gameplay. Overall, this game delivers in every aspect it should, making it an all-around amazing game.