Leaving aside the Broadway show that earned multiple Tonys by poking fun at Mitt's faith (an easy target, since no Mormon is going to, say, blow you up for your mockery), and given the fact that non-Mormons (like me) might have a hard time believing what the Book of Mormon (which I have read) says, why should anyone care that Mitt is a Mormon? Atheists and leftists believe a lot of dang-fool things that I think are utter crap, but I would never say that those beliefs should exclude them from highest office.
Let me rephrase that: there's faith that makes you a better person--kinder, more decent, more family oriented, more philanthropic--and faith that makes you act like a sanctimonious asshole. Every indication is that the Mormonism Mitt practises makes him the former, while the type of faith Barack Obama has embraced, at least during his Rev. Wright years, was far more problematic. Yet I don't hear anyone saying boo about that.
Now, if Mitt were into Scientology, then his "religion" would be of concern.