The C has more filters, more poly, more mod routings (i think) and it looks cooler.
More poly I find important actually as sometimes I make leads and bass that seem to eat it up really quickly. Couldn't imagine having less now.
The new filters are really nice. You can hear the 4-pole in action with the "heavy electric" demos I have on Soundclick. That filter isn't on the classic.
Bassicly, the price is a fair bit different too.