Well, if you liked GoW 2, you will like this game. It pretty much picks up a few years after the GoW 2 left off and continues the story from there. There are some new weapons and new enemies to ramp things up as well as a new and improved Horde mode that utilizes fortifications and boss waves. Then there is beast mode which contrary to popular belief is NOT a Transformers game mode in GoW 3, but the opposite of Horde with only 12 waves and a time limit.
If you are a completionist/achievement whore, you will have a miserable time with this game. If not, you will probably like it if you liked the previous two installments.
On a completely unrelated note, I see that you've played a lot of Arkham Asylum, but not Arkham City. I don't know if you are waiting for the world to end or something, but if you got ANY enjoyment at all from Arkham Asylum, you need to play Arkham City... like right now. Go, get it. It's cheap now, so there's no excuse. You'll thank me later.