(This is not exactly a comprehensive answer, but...) some optometry schools require microbiology and psychology as pre-reqs, which med schools (to my knowlege) do not.
I remember talking to a cousin of mine who's in med school right now, and he said that he's requirements included 2 semester of bio, chem, phy, and o.chem.
Whereas with optometry school there's usually a whole lot more required classes than that.