Ok, something I'll add for "public consumption" here, is if you've played Mass Effect 2, the way quests work is almost identical to that. You get choices to make, and fully-voiced dialogue (although the voice acting is even better in SWTOR than in ME2) to go with everything from both you and your companion, as well as whatever NPC you're talking to. Keep in mind that every NPC in the entire game that you can interact with has fully-voiced content. Vendors that you just sell grays to and crap don't, but they don't say anything via text, either. There is no such thing as picking up or turning in a quest in this game and not having full voice work with it, which is fantastic.
It's got great story in it that really ties in with the Star Wars universe, as it well should. Every class (not advanced class; but class, so 4 on each side) has its own class story with three sections, the final one finishing not long before you hit the max level of 50. If you've played Dragon Age Origins, you know what good story is from BioWare. (Dragon Age 2 didn't have as good of story, IMO; though it was still very, very good by industry standards.) It's even better in SWTOR. The story for every class on the empire side is great thus far, though I've not even finished the first (and by far, largest) arc on any of the characters. There is at least 200 hours of story (by that, they mean voice dialogue in cutscenes you listen to) for each class, which is an insane amount. You don't hear all of it because you make choices that then give different responses, but there is still an absolute ton that you do get to hear/see.
Your dialogue choices, most of which do not give you light side or dark side points, still affect your companion points. Every companion has their own personality, and what you do adds or removes companion points based on what they
think of your actions. Remember that you end up with 5 companions, (Bounty Hunter I think gets 6 total, but I don't recall for sure) and each one is different, including their abilities and the roles they can play. If you want to go pure dark side and choose all the most evil options, most companions will not like that; and you can even get some to literally just sit on your ship and cry, if you are cruel enough.
I believe everything I've said is already public info so it's not an NDA violation. There is so much more I could share, if not bound by the NDA; but suffice to say, if you like Mass Effect 2's style of game play in terms of the questing and voice work, you will love SWTOR. If you haven't played Mass Effect 2, then go buy it. It's available on Steam and Origin for $20, and it'll help you figure out if you want to buy SWTOR.
The replayability that this game has is absolutely ridiculous. There are 8 advanced classes on each side, (basically mirrors of the other side's classes, but with a lot of little changes and distinctions, which helps keep things unique) with each class having two advanced classes. Bounty Hunter has Powertech (tank and DPS) and Mercenary, for instance. (Healer and DPS.) You cannot switch between advanced classes once you make your choice. You start playing as the overall "class" (Bounty Hunter, in the aforementioned case) and then you choose at level 10 whether you want to be Powertech or Mercenary. Now, most of the quests you do leveling from 1 to 50 are world quests, meaning shared between all classes in your faction, but a good portion of the overall that you do (15% maybe?) is purely for your specific class, so if your first character is a Bounty Hunter Powertech and your second character is a Sith Warrior Juggernaut, 15% give or take (and that really is a guesstimate, it might be more, I don't know) is going to be completely different.
However...what helps add to the replayability here is that NPCs treat you differently, and actually respond to you differently in many ways, based on what class you are as well as what race
you are. Anybody who has read some Star Wars books knows that the Star Wars universe has a lot of tension between races. (I don't want to say a lot of racism, though it sort of fits the bill.) The same thing is in here. NPCs generally treat you with a scared respect if you're a Sith Inquisitor or Sith Warrior, but if you're a Bounty Hunter, they tend to treat you with disdain. When I say "treat you", I mean literally when you start talking to them, what they say is different based on your class and/or race. If you're not a human, some NPCs will call you an alien disdainfully, whereas a human would get a friendlier greeting; things like that. The amount of thoughtful detail that has been put into this isn't something you really think about, but it ends up making a huge difference.
What this means for replayability a lot, is you can sit and level a Bounty Hunter Powertech, and level up as a human and be a dick and choose a bunch of rude dialogue options and go for dark side points, then turn around and level a Bounty Hunter Mercenary, and even though they share all the same class quests because they're both Bounty Hunters, you can go light side and be nice and have a lot of different responses and reactions when picking up and turning in quests. Pretty much all responses you can give include a "nice" response, a moderate/neutral response and a dickbag/mean response. (The mean responses are oftentimes quite entertaining.)
I'm trying to think of what else I can say that wouldn't violate my NDA, but it's pretty restrictive. Bottom line is, this game's awesome. A lot of the actual abilities are WoW-like in nature, but honestly, that's just common for MMORPGs in general, even ones before WoW. The rich story and how there are various story arcs all over the place and stuff, make for a richer MMO world than anyone has ever seen, and by FAR. Some of the stuff you do for people is freakin' crazy...it's wild.
Buy the game.
Oh - one other thing. I like Star Wars quite a bit, but I'm not an uber-nerd for it. This game, if set in a random sci-fi world like Mass Effect 1 & 2 are, would still be amazing. It's not Star Wars that makes this game great; that just adds further to a game that is already awesome as all hell, so if you're not a fan of Star Wars, don't let that detract you from checking out the game.
Edit: another thought. Almost every MMORPG I've ever played, and WoW in particular, leveling is just a means to get to the max level so then you can actually "do stuff". In SWTOR, I'm looking forward
to just plain old leveling up alts. (Extra characters, for you non-MMORPG types.) It's crazy to think that leveling up each class, even twice so you can experience every advanced class, is something that could be enjoyable in an MMO - because nobody has yet MADE it enjoyable. Well, December 20th, somebody's making "grinding alts" an enjoyable process. I'm playing the Empire side, and I can't wait to just level up alts on the Republic side, as well.
Oh, by the way - all the flashpoints (4-man dungeons) are unique to each side. They aren't shared, like in most other MMORPGs.