Most ex-BMX guys I know end up being aficionados of stunts and highly technical trail riding when they switch to MTB, as opposed to endurance-oriented cross-country riding. That means something with a decent amount of travel, sturdy construction, and perhaps slacker angles than a race-oriented XC bike. Look for bikes in the "all mountain" or "freeride" category. These tend to be full-suspension, and therefore more expensive. You might also think about a "dirt jumper" like the Specialized P3.