I remember, having completed both KOTOR games twice, I went back to try and play the nasty Dark Sider and just couldn't do it. There was some really just petty, mean things you could do (like stealing a trophy from a widow, the only means she had of getting enough money to get out of her dire straits).
That said, I've been playing the Sith side specifically because I don't want any story spoilers when I make my first toon, a Republic Trooper.
That said, I will undoubtedly create a dark side character eventually (the Agent and Bounty Hunter interest me, and I have to admit to a certain level of interest in the Inquisitor story line I've seen so far in beta).
Like others have said, I expect there to be a heavy imbalance for pre-registration, specifically because you can only pre-guild one character.
Once the game is live and all 4 guilds are set up, I'm sure you'll see a better balance of Republic vs. Empire, and I wouldn't be surprised to see a bit of crossover on the US vs. EU as people start creating additional alts and are more willing to put up with some lag to spend some "face time" with their friends across the pond.
