I commute almost entirely on road bikes, but I am rebuilding an old Trek 850 as a bad-weather bike. My concerns with a MTB would be largely centered around the suspension, which adds weight to the bike and decreases the amount of energy going into the drive. As in most things, suspension is a trade-off. If you're commuting, you might look for an older MTB with a hardtail and a non-suspension fork. Disc brakes are nice, but hub/drum brakes, like the shimano nexus, are even better (less maintanence). I like the set-up of one of burley models: drum in back, disc in front. A bike set up soley for commuting, with fenders and racks, isn't a bad idea if you're going to do it regular-like.

But hey, you can commute on almost any bike. Get one, commute, and have fun.