I don't know about penalization. If someone leaves, however, they will be replaced. The warzones also have a thing where everyone's character is 'normalized' (increased hp/mana/whatever) so you could have lvl 10's fighting alongside lvl 30's. Arguably, this was done to make pvp more accessible, and more skill dependent rather than gear dependent. (world pvp is separate, and I don't know anything about it)
The mirror classes are:
Smugger = Imperial Agent
Bounty Hunter = Trooper
Jedi Guardian = Sith Warrior
Jedi Sentinel = Sith Inquisitor
I played the Smuggler character during beta out of the starting zone, and it was very good. I also played part of the Sith Inquisitor, and I really liked it as well. Bioware put a lot of work into the storylines so I imagine they will all be good.
Why is the acting better than in the prequel films? Because George Lucas wasn't involved--do you need more reasons?