Which class was super hyped, and became a let down for you?
For me it's the Jedi Sage/Sith Sorcerer, no contest. I'd heard so many people say they prefer the Sage to the Shadow/Assassin and I'm just completely baffled by this. There's no doubt that the Force powers are really cool, and being able to be a healer is always nice. Unfortunately, the class is let down badly by its low DPS & surprisingly squishy companions, meaning that unlike the Shadow who can often kill enemies before they can hurt you or your companion too badly, you are definitely forced to cast healing spells to win most fights (even if you're Telekinetics specced).
It is still a really cool class in its own right, but, to be blunt, having to constantly spam heal you/your companion to stay alive is no fun. If SWTOR's combat system wasn't so unfriendly to a "caster" ranged class or you actually had the DPS to kill mobs on your own, the Sage would probably be one of my favorite classes in this game. As it stands, I got mine to around Alderaan before giving up and rerolling as a Shadow, which I found WAY easier/less annoying to level up (especially KC specced with the healer companion). And this is coming from someone who usually prefers casting to melee in games like WoW.
In terms of storyline I'd have to say Imperial Agent. I will say that if you're a fan of James Bond-type themes you will probably LOVE this class, but I personally am not really into authority/military - from what I've seen I think I will like the Bounty Hunter & Smuggler storylines much better. But I think a really UNDERrated story is the Jedi Consular.