I commute year round in Toronto, which I think would be pretty similar to Boston. I think you would do well with a Hybrid, cross, or a touring type bike. Here's what I would look for specifically:
1. Needs to have mounting points to fit fenders. This helps keep your drivetrain clean, keeps salt out of your headset, and keeps a lot of gross slush off of you. Staying dry lets you cycle for longer comfortably, and lets you wear less clothing.
2. Fits bigger tires than a road bike, but not true mountain bike tires - Bigger tires (at least 32-28 mm wide) get flats less often and feel more stable in slushy/snowy conditions. You don't want completely bald tires like you would use in the summer . . . a little tread will give you some traction in the loose stuff. Mountain bike tires aren't needed most of the time in the city. They slow you down, the big studs are worse on ice than regular road tires, and the bike will put you in a poor position aerodynamically. I found that the really wide mountain bike tires were worse in slush than skinny road bike tires - the road bike tires punch through and get traction on the road.
We rarely have very icy days or freezing rain, so I'm able to get by with regular tires all winter. If it's too icy for my regular tires on the bike to handle the road, I don't really want to be out there with the idiots driving sliding their steel cages around. Depending on the amount of ice you get and how the road crews are at cleaning stuff where you live, you might want to consider studded tires.
3. Throw some studded flat pedals on. This way you can use heavy winter boots, hiking boots, waterproof boots, whatever . . . your feet are warmer, you'll get good traction on the pedals, and you can get your foot out instantly when you start to slide.
4. You want to be able to get into a reasonably good aerodynamic position. Winter means high winds. Cycling into a 60 kph headwind is miserable. The lower you can get your head and narrower you can get your arms the better it will become.
5. Tough wheels. My winter bike wheels take a beating. It's dark and there are invisible potholes everywhere (that multiply all winter long as the plows go by). I weigh more because of all the winter clothing I'm wearing. Your wheels are treated harder in the winter. If you get higher spoke count wheels you can snap a spoke and still get yourself home. Make sure you have brass spoke nipples (the salt will corrode aluminum ones into place)
6. Cheap components. Stuff just corrodes easily when it's hit with slush and salt regularly. Expect to need to replace some of it every year or two. (Chain, cables, cable housing will probably need replacement each season.)
7. Easier gearing than your summer bike. You will go slower in the cold, so you probably don't need a 52 or even a 50 tooth big chainring on a winter bike. Having a wide gearing range is great though . . . that 40 kph headwind on the way to work means a miserable easy gear slog, but on the way home is a wonderful and zippy ride.
8. Frame material - YMMV, but I'm afraid to use a steel bike in the winter with the amount of salt we get. Aluminum is reasonably corrosion resistant, and just seems like a sensible choice to me.
9. Brakes - Disc brakes seem like they would be awesome, but I've had really no problems running cheap v-brakes and Kool-Stop salmon pads.
I did a year of commuting on an old mountain bike, then upgraded to a hybrid . . . a Giant Escape which worked great for three years. This summer it has been converted to drop bars with bar end shifters, specifically to catch less wind while commuting in the winter.
