They have two servers, Algalon, which is progressive, as in you have to complete Naxx before Ulduar Opens, before ToC Opens, before IceCrown opens, etc...
That server is packed.
I dont' know for the other one as I don't often get to actually play.
Also, getting an actual 70 on the server, yes you can donate and get one, but you get one and thats it. For me, I would rather skill up, and play as thats what I loved most about WOTLK. Just getting handed a maxxed out character is something I can do with Blizzard now, and its BS IMO, and is ruining the game.