When you play with people you don't know you usually have to adapt to each other, and the circumstances. This may mean trying to go quicker, or the opposite, waiting a bit for people who go slower, or slowing a bit to make things go smoother.

Usually people want to do the dungeons as fast as possible, so as a tank I try to go as fast as I think the group can go, or myself. Only if you tank the same dungeon many times and it hasn't passed a lot of time, you know for sure what groups of mobs can be killed together and what isn't worth it, so I sometimes don't try to group if I don't remember well what comes next.

I don't care if people pull more mobs to make the groups bigger, it may make things faster, I tank them too and nearly always it goes fine.

As it has been said, you may meet nasty people. The great majority of people are nice, but some are not and there's the chance to meet them. Some claim for a speed run and cannot kill the mobs quickly enough for the tank to survive, or cannot heal, or cannot tank that many mobs.