I don't agree that Christina and Izzy are the mean girls of the hospital. I think that the lines that are drawn are not drawn by them thinking they're better than the others or what have you (mean girls style). Rather, I think the lines are drawn by the nature of their jobs. Addison is an attending. Callie is a resident. Izzy and Christina are interns. It's natural that you socialize with people at the same level as you. It seems stranger to me that Addison and Callie would be buddy buddy, or that Miranda speaks to attendings the way that she does, than it does that Izzy and Christina tend to segregate themselves from their superiors when they're socializing and complaining about work.