I pre-ordered on Wednesday through Origin, and downloaded the client while I was at work.
I didn't get into the early access until Friday afternoon, as a result.
I played a night elf Druid mostly in WoW, starting in vanilla, as a cat-form DPSer. I race-changed him to Worgen as soon as that became available. I like the stealthy "get in, kill things quickly, get out" sort of tactics, and the hybrid caster/fighter classes as well, which is why that worked better for me than going as a straight rogue or mage or something in WoW.
I doubt they'll add respecs for choosing the advanced classes, since that's really the class you're going with, and they just let you get through 10 levels before choosing. But they have WoW-like respecs for the talent trees as you'd expect. No idea how much they cost though.
My impression is that the Knights (both guardians and sentinels) are pretty melee-focused classes, with a little bit of close-range single-target force abilities (and saber-throws as well). But the Consulars are definitely more force heavy. The Sage strikes me as being the true mage class, since it's full of ranged AOE attacks and the like, longer ranged force abilities, plus it has the healer spec, though it still has some melee capability. The Shadow strikes me as more of a hybrid class, with close/medium range force powers, more melee abilities than the Sage, and of course stealth. BTW, you get your first double-bladed saber for free when you pick it.