Fuel Rod Cannon. It was apparently never available in MP before.

Scarab Gun. The legendary Easter egg comes to multiplayer at last.

Detached Turret. This one is really cool, it's called the GPMG in-game. No idea what that stands for. It's the mounted turret that's already present in the game, but detached and held in first person. Seems to function similarly to the SAW from H4. I hope the Covenant turret gets the same treatment.

Silenced SMG. As seen in H2V. The SMG but quieter.

Vehicles:

Falcon. As seen in the H2 Private Beta. Not the machine you're used to from Reach, it's in a very unpolished state and I can't wait to see the finished product. (If they're going to add this, it seems weird to not add the Halo 2 Beta Mongoose, since it was practically complete.)

Shadow. This is the Troop Transport version. Hidden in the files of, but not seen in campaign. It's unclear if the Vehicle Transport version will be available.

Gravity throne. This is the chair that the Prophets sit in in campaign. I can see some fun customs being made with this.