Are you a bad tank that doesn't know how to hold aggro? That's very important information.
Nope, not at all.
I'm also only talking about 4-man dungeons.
Here are some examples:
Big pulls: while I'm rounding up all the mobs there are some dds who think it's a good idea to go full dps on the first thing that gets into their range. Then when they aggro said enemy, instead of bringing it to where I rounded up about a dozen things, they run around doing whoknowswhat.
Fights with 2 or more mobs: if I don't mark things (I usually don't do that anymore besides in <50 dungeons, since you meet some really special people there) it might happen that dds can't decide what to attack and each of them pick their own target. This can become compliacted in situations where I don't have Shield Oath yet. If a melee aggroes the enemy it's okay, I just take it back. If a range dd aggroes, well that's where it becomes annoying.
Healer hotting before pulls and then running away from me.
I main BLM and when I notice I'll aggro something soon I'll move closer to the tank even before something happens. This seems to be a totally unknown concept to some. Instead they just start running away from me/the tank.