There's a pretty interested "movie" mode here, where you can record your game then save that file. You can then view it in the "Movie Theater" or share it with friends. It's kind of a neat tool, though I hope eventually you can watch the matches played out outside of your viewpoint.
I also wish there were instructions listed somewhere. I'm not sure what all the new gadgets do or even how to activate them.
Matches seem to be limited to 6 player, and I've only seen Slayer mode. There are 3 maps in total, all are new.
The graphics are interesting. "Next Gen" is so often now associated with Gears, that I fear people may be let down by what they see here. It's a much cleaner environment, with clearer lines. Because of that, it might not seem "Next Gen" enough to some. It really does look great, even if it is just an upscaled Halo 2. Everything has very nice detailing on it, though of course you're too busy trying to not get fragged to notice.
The matchmaking system is very similar to Halo 2 as well, and it's been very painless to get into matches so far.
Overall, very good stuff, and a terrible tease of things to come.