1. If someone leaves a warzone will they be penalized? Also, can someone replace them mid game?

2. What are the mirror classes for the non force users, Smuggler = Bounty Hunter, Trooper = Imperial Agent?

3. Any of the republic stories interesting?

4. Why is the acting better in this video game than in the prequel films? (Not sure if you can answer this, still it perplexes me)

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?

I dont know the answer to that question

Yes, gear and weapons still play a big role. A level 10 may have as much damage and health as a level 50, but not near as good weaponry or gear which usually ends with them getting raped which is one thing I don't like about the Pvp. If your in the 40-50 range you should never really be killed by someone below 30's unless your just thar bad.