The campaign is SO much better. The new cinematics, MUCH more action, nonstop fights, etc. are great!
I have also notice a lot of the tweaks. You are no longer stopped when stunned, you just lower the chainsaw or get slowed down. Grenades don't stun either, so no more sitting duck moments. You can also finally jump off cover more responsively and even cancel out of a jump from cover to cover. The little tweaks are noticeable, and they help A LOT. I don't find myself wall bouncing nearly as much either.
And no more one shot sniper downs makes the game so much better. Oh, and NEVER let everyone die so that it is AI bot versus AI bot. They are pretty good in a normal match, but 1-on-1 they usually play until the time runs down. 3 min matches are good though.
Overall, everything is improved. The larger color pallete makes it more appealing, the vistas (aka larger levels) are brilliant and beautiful, and the effects (bullet trails, gooier blood animations, destructible environments, more precise chainsaw animations, etc.) just make you feel like you're in the game. If you loved Gears 1, this is better in every way. It even has the flashback DLC so you can play some of your favorites from the first. Personally, I miss Rooftops, but I'm sure I'll find my new favorite.