Fit is the most important quality. For that reason, I only buy from my local bike shop because I like to try it on. Look for a stiff sole and quality construction. I like shoes with a ratchet-type mechanism because they seem to fit more snuggly. I've been a little disappointed with velcro-only closures, but I have a pair that I use for triathlons.

Lots of good brands out there - I've tried Sidi, Diadora, Shimano and Specialized. I like the Specialized shoes the best but that may or may not be the right choice for you. Anyone who thinks that one brand is right for everyone is delusional.