Too Long; Didn't Watch version: Gears of War 3 fixes everything that was wrong with the first two games. The Second-Act-Secret-Project-Ass-Pull-That-Invalidates-Everything-That-Came-Before? Gone. Survival sequences that exist to pad out the game, and in the process kill any sense of urgency? Toned down. Two-dimensional characitures for squad members (Hi, Cole)? Fleshed out and given depth.
The only real minuses I have against this game, is that it effectively completes the bad-ass decay of the Locust - with the notable exception of the Armored Kantus, and that the Lambent Stalks aren't as well implemented as the Grub-Holes were in the first two games.