Crowds (lack there of):
If you truly want to get away and enjoy the mountains then Big Sky is the place to go. We were there last year for a week over MLK weekend and at times while riding the southern exposure felt as if we were out of bounds, becuase we didn't see or hear anyone the whole way down the mountain. I'm not sure how they maintain the mountain as well as they do with such a limited number of people but I can't really complain about that. The only time I ever waited in line was for the tram and that was only about 15 min, every other lift including those at the main base area you could basically ride right on.
The terrain off the tram is awesome from the milder pitch of Liberty Bowl all around to the steeps of Marx, Lenin, and The Gullies. The "trails" off of the Tram and Challenger are mostly black/double-black and have everything from bowls, to trees, to tight chutes. Not sure what else you could ask for, there is enough here to keep you busy for a lifetime. As for the rest of the mountain there are some steep trails peppered about, but for the most part it is great for beginners and intermediates.
Yes it's true, to the average person the nightlife at Big Sky sucks. There is not a "town" where you can go to walk around, eat, and drink. There are some shops and restaurants scattered about the base of the mountain which is a short drive from the resort itself. They have everything you need there and thats about it. The bottom line is that if you are going for the off mountain attractions then skip Big Sky and leave it for the people who just want a great mountain to ride.