From what I've seen of bmx and mountain biking, they look very similar (they both have you interact with physical obstacles instead of just traffic and wind). I've never really mountain biked, but I'd like to try it out. I'd also like to learn how to bunny hop and whatever. They've both gotta make me a better rider overall, and they're just another fun thing to do on a bike.

Tricks on a fully loaded rig, though; they're tricky. My regular ride has front and rear racks, front and rear fenders, tool bag, etc. Trick's have gotta be easier on a BMX bike.

So, I'm thinking about picking up a used BMX bike. If I get a BMX bike, is it reasonable to go mountain biking on it? Is there anything about a BMX bike that makes it a poor choice to take out on a trail? What sort of problems am I likely to face?