What makes one Anti-Virus Better than another?

Stop me if I'm wrong here, but I view anti-virus protection almost as a commodity. Some people swear by one program over another, or will trash one program completely, but when you ask them about the reasoning behind their answers, they either mention that their current favorite catches all the viruses or that some program they had in the past had some major malfunction (Which more often than not turns out to just be a user error). I'm looking for real evidence here; real differences that truly distinguish different anti-virus programs from one another. For example, I don't consider "ease of use" a distinguishable trait, unless one program completely encrypts their platform making it impossible to use. Sorry for the rambling, but I have been asking this question for a long time and I really want to get to the bottom of this. Can someone please shed some light on this subject?