Honestly the decision to give-up captaincy by both Vaughan and Collingwood came as a bit of a suprise to me, but i don't think one can really blame the captain for their recent run of bad form, England have just lost their way badly since the Ashes 05, their players seem to get injured frequently (and the x-factor they had before is missing) and the selectors also seem to have gone bonkers post the Ashes, as they are happy to hand over an England cap virtually to anybody.
A captain is only as good as his players, and when Vaughan had a strong squad at his disposal he surely delievered the goods, i hope people remember him for all the good things he achieved during his term as the English captain, and not for his horrible final run.
Pietersen is the only choice atm, but i don't think he is really that good an option for England, he certainly seems to have a attitude problem, but more importantly the last thing England want is to see KP's batting getting effected by captaincy, imo in the longer run somebody like Bell or Cook might prove to be decent leaders.