I don't like having two of the same class. My pally stayed at 19, and my warlock is 90. I don't have a monk, druid or priest yet, nor a dk but he doesn't count for obvious reasons. As for tanks, it leaves me with either guardian druid or monk. My warrior is a lv1 worgen, gfed cuz you can't make lv1 worgen anymore, so he's not going to be lvled. Healing classes leave me with priest, monk and druid, but again, healing is not my strong point.
I'm far too limited in my choices as you can see, so I need to play one that the group will accept and I can dedicate to.