The big thing is - It was not the fuel. (Mr. Branson is apparently quite annoyed that everyone was blaming the fuel).
In fact, the engine and fuel tanks were found intact at the crash site. It's not the way anyone would have wanted it to happen, but I think this has proved that the hybrid fuel being used is indeed stable until properly mixed and ignited - and as an important part of the program is developing fuels for use in intercontinental ballistic transports that will land and take off from regular airports, I think this is a very good thing.
So. What happened?
We actually know what happened - but we're a long way from knowing why.
The co-pilot, either because he accidentally knocked it or because he somehow got confused on his checklist (this was an experienced test pilot and I'm inclined to give him some benefit) switched the interlock handle on the re-entry "feathering" system to unlocked.
Two seconds later, the feathering system did an uncommanded deploy.
Obviously, this should not have happened. The system has a two-stage deploy system to prevent exactly this - the feathering system activating during the burn, an event the vehicle had no chance of surviving.
In general, with modern airplanes, an uncommanded deploy - a system activating when the pilot did not touch the controls - is actually a software problem. However, it's far too early to know exactly what happened and it's not even official that the deploy caused the crash - but the chances of a spaceplane surviving its re-entry braking system activating with the engine on seem pretty slim.
Still thinking of the families of the pilots and everyone at Virgin Galactic.