Halo 2...
First things first.. all the new features:
Weapons - If it's a sequal, you gotta have new weapons. Halo 2 added quite a few.. and I they all are perfect.. not too powerful, not too weak.. and each weapon having it's perfect place, and place not, to use. With the exeption of the Beam Gun, everything seems like it would be quite balanced in multiplayer, although I've yet to play it. The sword is just plain cool, and dual weapons are just as fun.
Playing as the Covenant. First thing that came to mind was "What the .... " Unexpected to say the least. Although in my mind it kind of broke up the story a bit, it added alot to it. Having the (some) of the covenant and humans side by side was cool.
Vehicles. Nifty, 'Nuff said. The changes to the previously existing ones are warmly welcomed, and the new vehicles don't fail to blow stuff up easily, so they are OK with me.
The ending.. First of all, I would never have expeted it to end like that. My first reaction was "The games over?" The ending could have used some work, I didn't get a chance to say goodbye to Master Cheif. If the game could have ended in his hands, it would have been alot better.
That's all I'm going to say, I'm off to eat!