Depending on what server you're on you should be able to interact with everyone. If you're on the PvP server you could still communicate but Horde and Alliance always aggro on each other so you best be careful.
Jabe said the best statement so far. "If only Lucas went with Blizzard." Lately I have been really disenchanted by SOE, not the game mind you I still love that. I just feel like they don't care about adding more content for current players but rather bolstering their product for new recruits. Their last few statements have been cold and leave nothing to be desired.
But Blizzard, which is a company I have ALWAYS had great respect for since they included that extra, toned-down copy of Starcraft with the game so you could give it to friends. I don't care that they take a long time to put out games because they put a lot into them and for me its always worth the wait. With WoW I get the sense that they care about the content and like Keer said you get that sense that game seems finished, not rushed. Even if you massively grind to level 60 you have still only done about 25% of the game. There is a lot to see and do.
As for playing two games yes it is rough and I will not choose between one or the other. I am a hardcore gamer and playing Wow to me is like playing one of my console games. I like vareity and this is a good one. Paying for 2 games and 4 account really sucks though.
So keep an open mind and if you have a friend with WoW go check it out at his/her place. Just don't go leaving SWG or I'll hunt you down and gut you like a fish.
