First of all, Reach will include a new class: Medic. This Spartan comes packing a Dropshield ability -- the precursor to Halo 3's Bubble Shield.
But that's not even the best part: The new Firefight will have a host of customization options. Players can create their own tailor-made Firefight modes, changing the enemy types, weapon loadouts and other modifiers, including Covenant health and aggression. You could seriously create a mode where it's nothing but peace-loving Grunts, if you want. Then, you can fill out some info and share your custom version of Firefight through your file share.
Tonight's demo only has two playable maps, but we're working to confirm the total number planned for the final release.
One final interesting tidbit we'll leave you with is this: Bungie said Generator Defense in the Reach beta was actually a test for Firefight.