I don't BLM anymore but it was the first job I got to 80 and did any of my beginning 'higher end' content with, and the reason I think it is so well designed, aside from the mechanical aspects such as how its rotation works, is that the more 'secondary' gameplay aspects compliment the job. Specifically what I mean is that everyone knows BLM's difficulty comes not form the rotation, but from knowing a fight and timings, and the fact that the job is almost proc-less allows you to essentially never look at your hotbar and focus solely on what's going on in the battle. Compare this with an 'easier' job like DNC, which still allows you to pay attention to a fight and move around due it being ranged and cast-less, but require you to nurse the hotbars with your eyes while doing so.