This is very sad and my thoughts also go to the Surtees familily and his friends. It is a reminder that motorsport is dangerous and every participant can pay the price for it.
The brief of F2 from a design point of view was making it as safe as F1. So if Williams designed the car to that safety objective we have to consider that the accident could have happened in F1 as well. It shows that nobody can ever be complacent about safety and we must not listen to unreasonable demands that increase the dangers of the sport. Young drivers must also be screened for the necessary experience and discipline required for such high levels of motorsport.
It also means that the relentless work of the FIA, the GPDA and the engineers involved in motor racing must continue. I hope that a very dilligent and thoruogh investigation will be executed and if necessary improvements to safety will be found.
Edit:
http://www.youtube.com/watch?v=aTDiYS1NVW4
It appears from the race footage that Surtees was first hit by the wheel and then went full speed into the safety barrier. So his head and neck was probably exposed to multiple severe impacts. I do not know how good the safety standards are at Brands Hatch but I'm sure it isn't on F1 level in terms of run off and barriers.