Will there be a mechanic for switching from hero to villain or vice versa?
No. You make the choice of hero or villain at character creation, and it can't be changed after that. This is so it is clear to other players in PvP who is on what team.
Will current storylines in the DC comic universe affect the MMO?
The game will launch "pristine"; that is, the DC Universe in the game will be the popular, well-known one. Wonder Woman will be in her classic costume; Bruce Wayne will be Batman; etc. From there, storylines from the comics may start altering the MMO universe, but at least from the get-go, the game will start with a clean slate.
DCUO will be getting its own bi-weekly comic as well.
What places other than Metropolis/Gotham city will we see in the game?
Iconic places will be playable -- such as Arkham Asylum, where you can let people out of their confinement, and those NPCs could then help you or hurt you. Ace Chemicals will also be there, and in that place you can hit people with a bat for funny effects. Even further out, Smallville is in the works, where Lex Luthor and Doomsday will be up to no good.
Plans also inlcude a playable area on the moon, a volcano with angry, intelligent albino gorillas, and the mysterious Area 51. In the last one, Brainiac is on the attack. Other well-known characters will show up in these areas. The Scarecrow, Mr. Freeze and Poison Ivy were mentioned for Arkham Asylum, and Sinestro along with the Green Lantern cops at Ace Chemicals.
Not all of the locations for adventure can be found on a map. One of the coolest adventures they talked about sends you inside Raven's mind to fight her tormented-soul-self. The other Teen Titans join you to help bring their teammate back from the brink.
How many characters will each player get per account?
Not announced, but enough to explore both hero and villain storylines.
Can you have a Sith Lord character?
Technically, yes. The character generator is advanced enough for you to get pretty close when trying to create popular characters from other universes. However, what Lucasfilms' lawyers think about that question is another matter...
And with that, the panel was over. They showed the Blur cinematic video Who Do You Trust? one more time; all the panelists posed for pictures; and everyone in attendance was given a code to get into the upcoming beta. I'm pretty sure I saw quite a few of them at the DCUO demo stations on the show room floor trying out the new Play as an Iconic game feature that was just announced.