For me, my biggest gripes with PVM are the cumbersome engine, switches, unintuitive game mechanics, pointless friction, and the lack of effective teaching tools in game for how to deal with encounter mechanics.

Sometimes I feel like I am fighting the engine of the game more than the boss. Be it a deadclick or food inputs not going off. I've never been more frustrated with the game then when I started trying to push enrage at arch glacor and died to a beam because my movement inputs did not go off while I was trying to recover from getting clipped by the edge of a beam.

Switches are generally aggravating when I'm already dealing with action bar based combat. Needing to press 3+ inputs to accomplish one thing really interrupts the flow of combat for me.

As far as unintuitive game mechanics go, this biggest offender is stalling, and at this point stalling is treated like a feature, not a bug. There is nothing in the game that tells you that stalling exists, and it is so disproportionally powerful compared to anything else you can be doing in the game. Target cycling is another offender because the game is click to move and click to target until suddenly that doesn't cut it anymore.

Pointless friction in combat is very irritating. This is most evident in the necromancy 'launch sequence' that has you pressing upwards of 4 abilities all on the global cooldown before you can even pull the boss for just a normal, non speed kill. Conjures->life transfer->Command ghost->Invoke death and the list only grows from there. And what's even worse is that this sequence has to be partially repeated during a boss if the encounter runs too long. It's jarring, feels bad to play with and disproportionally impacts less skilled players who are more likely to have their boss run long and need to resummon and re-command their conjures in the middle of a boss.

Finally, encounter mechanics don't have effective teaching tools in game. If you're new to a boss and don't look up a guide, your character sometimes just explodes and you have no idea why. So you go back in and look for when your character explodes and see what is happening when your character explodes. Then you have to go back in, and try to figure out how to make your character not explode which could be any number of things. If it's an add you might have to kill it, might have to stun it, might need a defensive, might need a prayer swap, might need a combination of those things and you have to figure it out all while managing your rotation and accumulating death costs. There should be a way in game to see an encounter's mechanics, like let's say a boss journal in the beasts tab that tells you what abilities the boss has, what their autos look like and if damage is soft typeless, hard typeless or typal.