I made another run into Kara tonight. We were jamming. I was working with yet another different main tank though. One of the hardest jobs for an off tank (as I usually am) is to coordinate their pulls with the main tank, coordinate aggro with the main tank, etc. So I have to try and figure out the habits and style of the main tanks as quickly as possible. It can be a steep learning curve. Thankfully most of the time I'm running with well-equipped and experienced guild members, so rarely does the process result in a wipe.
I'm curious though....do the larger more established guilds run two tank teams? I know I would be much more effective if I was constantly working with the same main tank all the time.