"In June, 1974, the FIDE Congress in Nice approved the 10-win regulation and the elimination of draws from the scoring, but imposed a 36-game limit and rejected the 9-9 proposal. On June 27, 1974, Fischer sent a telegram from Pasadena, California to the FIDE Congress:
As I made clear in my telegram to the FIDE delegates, the match conditions I proposed were non-negotiable ... FIDE has decided against my participation in the 1975 World Chess Championship. I therefore resign my FIDE World Championship title.
--- Source http://www.chessgames.com/perl/chess.pl?tid=54487
I hate to even slightly agree with GreedyPawnGrabber on anything, but I'm not so sure that Karpov should even be considered an underdog.
Maybe it's because I try to have a global view on history and current events that I never raised Fischer high above the other greats... In the end the match never happened, we have no way of knowing how it would have played out; and comparing games against Petrosian sure won't help.
At least I agree on comparing games against Petrosian. Karpov and Petrosian's styles were very similar (positional play). There it's much more difficult to win games.
Fischer and Kasparov also had similar play. Both liked active play and knew very well how to use initiative!
Euwe wrote: "FISCHER liked lively positions."
(Source: From Steinitz to Fischer, Chess Informant 1976.)
Unfortunately we don't have those characteristics from Euwe for Karpov and Kasparov.