Dear Sir Frank Williams is known to make some smart gambles. But also known to make some blunders (remember HHF, Zinardi, Ralf)... so using Williams as a gauge is not quite as clear an indication.
Objectively, on a rank of 1 (worst) to 5 (best), i'd but Mark somewhere in 3.5 range. He's much like Jarno Trulli in 2003 - 2004... a fast and impressive qualifier, but isn't quite as strong in the race. His ability to race wheel to wheel is also open to questioning. But unlike Trulli, MW is far more strong-willed and confident.
I won't go on to say he isn't WDC material. Any driver nowadays just need to be decently quick and consistent...and he can win the WDC as long as the stronger drivers are in far weaker cars. And decently quick Mark Webber is...all he need is to tune in and further improve his consistency (ie no unneccessary mistakes) and just wait for him to get a car better than Michael, Kimi, Alonso, Montoya.
Right now, i rank him between the likes of DC, Ralf and JV... the range of decent and just a bit more drivers. Time isn't on his side, and as usual, i'd love to see him really show his stuff next year. There's nothing better than to see a driver perform above the ability expected from him.