Yeah Boomkins are complicated...no gear support pre-bc and post-bc it's still a learning curve in how to handle yourself(Abusing roots, knowing when to fight, when to unshift to heal, when NOT to unshift to heal, finding out that Nature damage(wrath, IS) is less likely to break roots while Melee/Arcane dmg(or any other player's non-nature dmg) WILL break roots 99% of the time, etc

I personally started with mages as their spells are simple to start, but I only used them to learn the control of multiple chars; if you want a lvling set without practice first, go with Warlocks. Pets to cover you while you learn and just to tank in general + doesn't require you to face the target most of the time to dot them, fears to save yourself, SS/Healthstone wipe prevention, mostly pros, few cons for a starter