BLM is a pretty easy class to get used to and do moderately well with. You use a fairly low number of skills regularly and essentially can't run out of mp (though a major screw up could leave you unable to attack for up to 10 seconds). Like others have said, it's a class that can be played in a very simple manner to get by and scales up a lot based on skill and experience. The 'rotation' is simple, the DPS gains are mostly from knowing when to break out of the rotation to do something slightly different, as well as knowing when you'll need to move, doing it pre-emptively (when possible), or negating the attack with manawall/ward.