How do you fix something no one thought was broken? Gears of War 2's Horde mode was such a success that it propped up the game while its competitive multiplayer floundered under online matchmaking issues. It's also one of the most imitated game elements since third-person, cover-based shooting became the standard for shooters, starting with the original Gears of War.
While the general cleanup and improvements Epic has made to Gears of War 3--in the form of more responsive controls, retweaked weapon balance, and newly-colorful visuals--go a long way to making "Horde 2.0" (as Epic is calling it) a more enjoyable experience, there's more to it. It's said that life needs more to it than survival, and Epic is taking that to heart, injecting Horde 2.0 with objectives, goals, and benchmarks. They're adding a point beyond "struggle to survive."
They're also adding tower defense. And Brumaks.