Air? Pollution? It's just city air--not noticeably bad, but not fresh mountain air or anything. Boston/Cambridge is a great college area. As far as crime, I'm sure there are bad areas of Boston, but the MIT campus and surrounding areas of Cambridge and Boston are pretty safe.
The weather might bother you if you're from somewhere warm. I don't mind the winter myself, I just don't like it when it stays cold all the way up through April.
Personally, I think Massachusetts is a pretty crappy state. Going to college here is just fine, but I'd hate to live here.