Theres not really much to say other than it's still very much halo. If you like the other games you'll love this. The campaign is pretty good, firefight is much better than ODST. You can do split screen with 2 people per screen, but system link a couple of xboxes together and you've got the best way to play. The only really annoying thing is that the space level that's in all the trailers is the best bit, but way too short.
Forge mode and custom games are awesome. You can just build whole levels in forge. I've had awesome fun trying to build a pair of spaceships floating in the sky with teleporters and mancannons for capture the flag. It's way more customisable than halo 3 from what I can remember of that.
The custom armour thing that's the new big deal seems really cool at first, but because of the way it's set up it seems like a lot of people will end up looking rather similar, it's still a nice touch though.
It's still kinda lame that they feel the need to remove things each game. Those cool brute motorcycle things are out, replaced with an elite hover bike that basically does the same thing but not as cool. The halo 3 helicopter thingies are out, with a new much slower version to replace it. In fact, most of the new weapons and vehicles seem to be replacements for ones from the old games.
Erm.. what else? You can't dual wield any more, the armour abilities are really fun (jetpack is the best IMHO followed closely by hologram) and the old pistol from halo 1 is back... as overpowered as ever.