What I consider to be major upgrades from the SB-600:
- Commander capable (up to 2 groups). Now there are four commander options for Nikon (in addition to the popup flash): SB-700, SB-800, SB-900 and SU-800.
- Head rotation: 180 degrees to both sides according to Nikon.
- Zoom coverage up to 120mm.
- SU-4 optical slave mode. See here.
- Other features similar to the SB-900 such as the interface, light distribution pattern, support for gels, overheat protection, etc.
- Power - the press release gives a GN of 38m at 120mm zoom, 100 ISO. I'm guessing the 35mm zoom is 1 stop weaker, meaning about 27m, which is a bit weaker than Nikon's stated GN for the SB-600 (30m at 35mm zoom, 100 ISO). The cynic in me thinks this is intentional on Nikon's part to force us to buy more. :)