I don't know how much of these are 'distinguishing' characteristics, but here's a little bit of a feel for mac:
It's one of the most liberal, but not in an extreme way.
It's very compact, since it's in a metropolitan area.
Exceptional econ, chem, international studies departments.
Good food.
The 'jocks' are quite smart and friendly— just not quite as good at sports as at other schools

(Less jocks overall)
Preppy clothes are rare
No expectations of fashion
Only a small number of 'hardcore' nerds, by which i mean very poor social skills and constant video game playing— most in math/science are fairly well-rounded and social.
Cold
Practically everyone has some experience abroad.
There are some cliques (mostly internationals and jocks) but the majority of students mingle all over campus. (in the first year at least)
A lot of procrastination
A lot of people who don't seem smart but then they blow your mind (i.e smart hippies)
Humanities courses can have an immense workload. Math/science tend to be more reasonable. Humanities tends to produce somewhat inflated grades. Chem and econ produce averages slightly below 3.0.
There's a good deal of intellectualism but most of the year people have too much to worry about to get into long 'deep' conversations.
There's also a lot of groupthink.