I've experienced this from multiple angles, having originally leveled a healer to 50 at launch and then coming back to the game to level a DRG who has MRD alt at 29.

I had some really bad experiences with tanks who could not generate aggro as a WHM, because they tended to be the same ones who did things like stand in 3 red aoes stacked on top of each other (thinking of the 3 avis pull in vigil) requiring me to heal my ass off. The way WHM is built, at least back in 2.0 when I played it, you had to be careful to only heal as much as you needed to or you would pull hate so if a tank sucked at aggro and sucked at dodging it became a) let them die and take hate or b) divine seal, heal my ass off, and then take hate anyway.

As a DPS, I have taken hate multiple times and sometimes its my fault (i.e. I tend to do as much dps as I can in any situation; a bit spoiled cause the tank I played with in SWtOR had awesome threat) and sometimes its just really bad tanks (looking at those guys I've pugged t4 with who are i90 and cannot handle the dps even when one pair is on one tank's knight and the other is on the other tank's soldier). When I learn a tank cannot handle me or they obviously under gear me, I do hold back and if you don't as a DPS that is just obnoxious. Even if it is that the tank is not playing optimally, which is often the case, you are in a group and you need to play to that group.

As a tank, I have one dungeon run that really got to me. I was a 28 marauder in hauke mannor with two dps level sync'd from 50 to 32 both with their job crystals. They insisted on dpsing separate targets and sometimes would even pull additional targets by accident and go straight to dpsing them before I could establish hate. This was especially bad on the bosses where on the first one the healer let me die saying "oh i thought i was a mnk, forgot to heal," and then when i'm ressed the dps dont let up for a second to let me get some aggro back. I get it back but never so strongly that full thrust wont pivot it off me for a second. Then on the last boss, I tried to tank the adds when they popped but the dps just stayed on the boss and took threat. It was just a really awful experience, even if I managed to keep threat 90% of the time on the mobs it was tough to do so.

That is a time where I wish the dps would have realized a) I dont have defiance yet, b) they have their job crystal main stat bonuses, c) they are in level sync'd gear sync'd to i think it is 32 so four levels above me, and of course d) I am not an experienced tank in this game who can take all their crap.

Idk in general when pugging you are going to run into a lot of annoying people whether because they are used to doing things differently or with particular people, or because they are genuinely just obnoxious people.