The job is is very simple to play when you don't have to move. The rotation is rather basic and takes very little practice to get used to.
BLM difficulty comes from being able to never loose enochan while moving while keeping the GCD rolling. The game provides you tons of tools to do so and BLM having no mobility is just something someone who hasn't played BLM since HW would say. BLM have TONS of mobility. But it comes in the form of proc and ogcd that you need to manage (which requires good understand of the encounter when you reach higher level)

So the BLM is the incarnation of "simple to understand, hard to master". It is the job with the widdest game between the "bad" and "good" blm. Meaning that a bad blm will do much much lower dps than a good BLM.
When it comes to the rotation people already explained enough so I won't bother repeating ^^

As a side note, BLM is a VERY rewarding job to play.