OP is from Boston, so am I. my winter tips:

with the proper gear, I find I'm comfortable down to about 15 degrees. then the real problems become
1) piled snow on the side of the road leaves you nowhere to ride
2) ice

#2 is addressed with studs, as others have said. #1 is a bigger problem IMHO. but there are a couple of routes that are plowed reliably. the best of these is the Southwest Corridor from Columbus Ave down to Forest Hills T.