Epic has done a great job of fine-tuning everything. The roadie run feels faster, the cover system snaps you into place nearly perfectly, and the bullets actually go to where they should. It's a bunch of minor improvements that have culminated in making the game much more enjoyable.
Gears 3 has undergone a pretty substantial makeover. The developers gave Sera an actual, tangible feeling of being alive, and being particularly desperate. The visuals are very crisp, and when you combine them with things like fractured lighting, or the tendrils of billowing smoke in the background, it really pulls you into the game.
In Gears 2, not only can your AI companions revive you and get you back in the fight, but you can tap "A" in order to crawl to them more quickly. It makes life a little easier, and the game a little more fair on the tougher difficulties.
Weapons have been balanced more properly: the shotgun was made more powerful, the pistol got a little kick in the power department as well. They've also added new weapons, the flamethrower and the mulcher (a huge chain machine gun) are the two most notable additions.
The flamethrower is great for burning down Locust that get too close for comfort. The Mulcher, though it takes a few seconds to get going, can tear through even the most powerful enemies with ease.
The cover system has been revamped so you cling to cover more accurately, meaning you're not going to be catching as many stray bullets for no real reason as you would in the first game.
Gears of War really puts the pedal to the medal in combat in more ways than one. Yes, it's fast and thrilling. Yes, it can be difficult (especially at the highest difficulty setting, it's a monster). However, the combat itself is just satisfying. When you blast someone with a shotgun, you feel like you just blasted someone with a shotgun. Likewise, revving up your chainsaw and massacring locust just makes you feel powerful.
The actors in Gears of War have done an outstanding job. You can really feel the tense air about the game, the desperation in some of their voices, and the will to survive in others. Little things like this really help to draw you further and further into the game as you progress.
Every aspect of the game helps to flesh out the story being told in Gear of War. Objectives and environment plays a big role here as the player learns more and more as they progress, but not though overly explanation based cut-scenes but through the game itself. Each objective the player needs to complete brings them a bit further in the story and in understanding why what is happening. This helps to make the player feel more involved, especially when expanded upon through well crafted environments. While the world in the game may be outlandish, it feel believable because it has all been presented well to the player.
With the re-vamped version of the game, as well as all the newer iterations (Gears 2, 3, etc.) there's no online multiplayer following. However, co-op campaigns are still a complete blast.