I main a 110 BRD, but secondary a 110 PLD (WAR is also 110, but I'm not as fond of WAR as I am PLD).
Having done BRD for much of the game, I have insight on pulling aggro from other tanks, as well as losing aggro on PLD to people who can't wait for hate.

A good tank will know how to hold aggro on all enemies, and if there is a struggle somewhere, a good tank will know to mark priorities for people to follow. At that point, it is up to the rest of the people to follow said marks. This may occur because a tank is lower geared than the rest of the people, or the other people are pulling hate first.

Also, a good tank will cycle cool downs when necessary. If a healer needs a break to save someone else, a clutch hallowed ground isn't a bad idea. In raids, I'll pop this if two people are down, that way the two healers can get them up without having to focus on me for a few seconds.

Knowing what your healers are capable of will help determine your capabilities as a tank as well. For example, if I have a healer from my raid group who is 110 BiS WHM, I'm inclinded to pull all groups before a boss encounter. If I'm running with a healer who is in only myth or soldiery gear, I'll do one to two group pulls.

Basically, a good tank needs to be able to gauge a group, and move from there with it.