What is a mountain bike? Good question, with the new freeride Bikes, XC bikes, Downhill Mountain Bikes, ETC.
As far as I am concerned, A mountain Bike has to be capable of the riding that an accomplished Mountain biker can throw at it, within the bikes capabilities. If you are Downhill, you will not often go up many hills. If it is a freeride bike, then you are not going to do many 40 mile trips. If you are XC then you are not going to emulate Hans Rey in the local shopping centre. If you are a complete and utter nutter then you are going to try all of these things on the one bike. Good luck to you, but one thing that all these different styles of Mountain bike have, is a friendship and admiration that all the owners have for others in the different disciplines. That even comes down to the beginner that can finally do a bunny hop, the novice downhiller that has finally missed the tree that everyone else hits, and the 20stone, unfit git that finally manages a 10 mile ride off-road without walking.