I think Game AI is one of best features of Halo3. Halo3's Game AI is certainly improvement from Halo2, which developers spoke about Halo2's Game AI in GDC.
Video: Halo3 my play video : stage 1 top view
Enemies' behavior is smart and is governed by a leader of a group, to shoot you. They use landform well to hide them from your shoot. Their behavior is smart, not mechanical. It's good idea to confuse enemies, because their responses are same as human beings. You have to play single mode as well as online mode.
Levels are very good. In single mode, enemies keep good points of landform to hide and shoot. But you may look at best point for counter attack.
Theater and Forge
Theater and Forge are really excellent. Many many users think these features are really excellent, and enjoy these every night. Implementing Forge means that users have great creativity, like MOD. These features will become common features of most games since now. FPS users (and gamers) like both of playing and creating. I think these two factors are the origin of playing. People create game rule and play it. That's human being's history. Card game and other traditional games indicate this theory.
In Halo3, we create new rule by Forge and share it in the net. Yes, in Halo3, you can share your replay, your screenshot and your Forge data with other players. Such features become common features these days.
This is the best game of the year. And you can enjoy this for a long long time. Even if your wife keep a close grip on your purse, you can enjoy a game for 3 years and greater by only Halo3 if you got Xbox and Halo.