Oh I forgot to mention.
The game encourages alts, in a few ways.
The Empire and Republic each have 4 classes (each have two Advanced Classes, but their stories are the same) and each class has it's own, personal, unique story-line. That's eight story-lines. Oh, and you can have Republic and Imperial characters on the same server.
I just hit 50 tonight, and I just became a Darth. And a lot of awesome stuff happened from 1-50, lol.
Also, once you finish Act 1 of your first character's story (around level 30-33), you unlock your Legacy. As of right now, all you do is create a last name for yourself, but Bioware has already stated the legacy is pretty much in it's infant stage atm and really isn't even implemented yet.
The neat thing is, though, that once you unlock your legacy on once character, you've unlocked it for every character you make on that server. You get a separate experience bar that is shared for every character you make.