1.The magical switch to activate the Halo 3 beta.
2. The remaining 100 Achievement Points.
3. The new multiplayer modes, 'gang reset' option or planes.
Here -- according to Xbox Live Director of Programming Major Nelson -- is what the update does do:
1. Makes some small technical fixes to "ensure a smoother online gameplay experience."
2. Increases the number of gang-controlled vehicles. ("Repo Man" Achievement hunters rejoice!)
Sure, you could view this as a disappointment if you were hoping for some of the above Crackdown updates. But we eternal optimists choose to look at is as just more things we still have to look forward to! Huzzah!