I aksed this same question to a friend of mine who owns a small chain of music stores. This is what he told me.
MIM: lower quality woods, different fretwire, usually chinese made electronics, plastic nut, in general lower quality parts. Woods might be the same types as those used in MIA, such as swamp ash or maple, but the wood isn't of the quality.
MIA: better quality everything, better woods used, USA made electronics, bone nut, better bridge, etc.
I have played MIM, MIJ and MIA basses. In my opinion, there is no comparison. The MIA basses are made much better and sound better. The MIJ basses are a cost effective alternative to MIA but I will never, ever buy an MIM bass.