Depending on where you ride you may get significantly less miles on an MTB chainset (middle and granny rings mainly) than a road one.  I ride in the North West of England and the gritty mud grinds the rings right down, particularly in the winter - can spend a lot of rides being serenaded by the expensive sound of your chainrings being worn away.  I agree with Dirt Diva about the BB also.

 The Shimano chainrings I've had have worn out much quicker than the Middleburn or Race Face ones I've had.

As julbags really, the middle ring can wear out really quickly as it's the one most used.  Then the rear cassette will wear, and the chain...

I seem to go through a lot of SPDs too.

Not needed to replace a BB on my MTB.  I've gone through a BB on my road bike quicker (campag rubbish...)