I have two pairs of SIDIs, neither of which are carbon soled, so for me, fit was the only thing (well, and price) that I cared about. There aren't too many other bells and whistles, at least not in my estimation, that are deal breakers or makers for me. I have a narrow heel, and SIDIs are the only shoe that works for me, although I wish the SIDI toe box was a little wider.

With fit, you want a shoe that doesn't slip, but isn't too tight either.