BLM is very fun to play, especially when you get Flare at lvl 50. BOOOOM!!
I find BLM easy to learn because it's all about casting as much fire spells to use your mana then swapping to ice to regain mana. The spell interaction with the two stances change as you level up so theres a learning curve there.

The hard part to master is learning to use your procs to instant cast spells so you can move around when rquired and not interupt your casting.