Let's see, let's see...Where to start...Anywhere!
Freakin' Vehicle Lowdown:
Plenty of new vehicle goodness for you guys to be enjoying.

Mongoose - Duh, the only new vehicle that most even know about.

Brute Chopper - A one-manned ground vehicle that shoots out exploding rounds. It also has boost and armor to make it a tough little opponent.

UNSC Hornet - An awesome little aircraft that takes on Covenant Banshees in Campaign mode, possibly in multiplayer as well. Appears to be three-manned, one pilot and two people on the sides as gunners.

Brute Prowler - Sorta looks like a jet-turbine on a sled. It can hold a driver, a man on forward mounted turret, and two passengers. Probably a cool replacement for the...Uh...That red thing from Halo 2 I didn't use...

Variation of the Warthog - Yeah, this one looks pretty fun. The turret in the back was replaced with space for extra troops (Looks like three extra), and yes, you get to use it in the campaign mode.

UNSC Elephant - Dear God, this thing is big...Bigger than the Wraith. It's in the multiplayer map known as Sandtrap, and it's a mobile base with a couple of turrets on it and plenty of troop carrying capability. No word on campaign use yet.
Weapons for the Killing:
Lots of new weapons that we weren't told about as well...Extra things too...A lot to remember when playing multiplayer.

Brute Mauler - Oh, man...This looks fun. A dual-wielding shotgun. Not as strong as the shotgun of course, but damn fun to use.

Gravity Hammer - Le gasp! Yep, you can use this in Campaign mode to bash stuff out of your way with ease...like vehicles...Oh, and the shockwave from the hammer will send thrown grenades away from you.

Radar Jammers - Mostly useless in the Campaign mode, but great in multiplayer. It'll trick enemies with false radar images and fake shield tricks to simulate enemy attacks...Yay!

Personal Shield - Like the ones that you always see stuck to the ground that just sit there...Yeah, you get these as well. Protects against attacks from one direction. Stand behind these and fire through them at your enemies, while their attacks do nothing.

Regenerator - This instantly recharges downed shields...The opposite of the Power Drain, then.
Flare - Blinds your enemies, great for taking flags away from blinded foes. Mostly in Campaign, but sometimes in Multiplayer.
Fourth Grenade - No word yet about this little secret, but there were plenty of hints about it. Keep guessing.

Old Pistol - Yeah, it appears to be back. It's more realistic now (Thank me), so no more whoring out of that like in the days of old and shooting people from miles away in the head with it. It's a pistol, it should act like it..Good dog.

Plasma Sword - Yeah, it's in there too. Can't change that thing too much, it's still powerful...Let's see how it likes dual-wielding shotguns?
Other Extra Goodness:
How's about some customizable armor in multiplayer?! Done.
Attempt to be even more unique in your world of killing as you strive to be the best. You can customize things from your helmet design to your shoulder armor patterns. Both Elites and Humans look...Pimped out? I rarely use that phrase, so don't love it. Check out a few renders of different armor designs here as well as a picture of the Brute Chopper in the back.
New modes for Multiplayer? Hellz yeah.
Zombies vs. Humans? Oh, thank you..
Customizable maps and game types? Of course.
The Campaign mode? Beautiful.
Newly applied graphics and awesome backdrops for all. The enemy AI's are more intelligent as well, and battles will be allowed to become bigger and bigger with more and more targets on the battle field thanks to the 360's added power over the Xbox.