I also agree with Zach. The Sith will more than likely attract some of the more 'kiddie' and (*shudder*) 'emo' Star Wars fans for the desire of being evil, mean, and killing babies. I fear that this could also cause a lot of problems for Sith players trying to group and do quests together. I foresee a lot of "lolz! i betrayed u!" Hopefully there will be something to avoid grouping with these kind of people.
I definitely plan on making some Sith characters (after reading the Darth Bane series I can't help but have a different respect for the dark side), and as such expect people to back stab me and try to take advantage of me if the opportunity presents itself - however, do it in a tasteful way, don't just do it because you're an ****. Maybe that's just too much of the role-player in me coming out though. I could even see new players looking for help having a hard time finding it due to everyone around them being 'evil' and trying to build power for themselves.
I see the Republic having more mature players, with people trying to help each other and overall being more team and group oriented. The only issues I could see here are kids just wanting to be running around with a light saber while still being a good guy. I think the biggest issue for the Republic being new people wanting help, begging for it, and being extra discouraged when a "good guy" doesn't go out of their way to help them.
Overall I think force characters will have the lowest average maturity - there are going to be some of everybody playing these classes, just because they are such a staple to the Star Wars universe, they are going to attract everyone.
Obviously, this is a lot of speculation to be doing so early on, it is just based off of what I have seen from the few MMO's I have played in the past. But, as constantly being pointed out: It's fun to speculate.