Diablo 3 has just launched patch 2.1. By and large the patch does a lot of good things but it is also illustrating some real problems that appear in nearly every RPG type game out there. In particular there are always tremendous issues with pets and pet classes that constantly plague these sorts of games, mainly that either the enemies are capable of killing the pets in which case the pet class is nonfunctional or the enemies cannot kill the pets in which case the pet class is invincible.
Player toughness is not nearly so binary because players do things like crowd control enemies, position themselves so that only a few enemies can hit them, kite, use defensive buffs, etc. Because of all the different tactics players can employ to trade off damage for survivability there is a wide range of monster damage that is workable. Pets can't do any of those things. They just stand there and get beat and either they are tough enough or they are not.
I am running into this issue with my Witch Doctor right now. Generally speaking the changes in 2.1 were an outrageous buff to me - several bugs were fixed with the interaction between Fetish Sycophants and Fetish Army, pets occupy less space so they can swarm more easily, and Life On Hit was massively buffed which is insane because I was already stacking a lot of it. I can safely say that my health and damage are both effectively doubled. I can handle the highest base difficulty without much worry and I am collecting new gear and cash at a fantastic rate as all I need to do is follow around my swarm of evil and watch them maul the enemies.
However, when things go bad they go really bad indeed. If the enemies can actually deliver enough damage to kill my dorks my army falls over very quickly and then I am left in a completely untenable position. If I summon more dorks they die rapidly as they are outnumbered and building back up to a critical mass is extremely difficult. I can potentially run back through the dungeon to hunt for weak enemies to farm to get my army back up to size but that isn't a very good strategy for a timed run, or indeed for any sort of efficiency. Doing so also assumes I can survive a situation where I have to take hits in the face instead of relying on my meat shields.
It feels a bit like launch day of the original D3. Pets were really excellent in the beginning, both tanking and beating down like champs. As soon as the challenges ramped up a bit though they were utterly useless. Right now pets are really good for the base game but as soon as I end up running through timed Greater Rifts which continually ramp up in challenge I am going to hit a pet wall where I simply can't keep them alive and no more progress can be made. I haven't hit that wall yet but it is clear it is coming and there is nothing I can do about it.
I won't trash on the developers for this situation though as the model for pets that D3 uses can't avoid this problem. When pets are expected to be very durable and not die this is inevitable. To get around the pet problem what we need is an entirely different vision for pets. Pets need to be something you constantly summon because they are primarily an offensive force and they only last a short time. They should not be so durable and numerous that their boss can ignore all enemies most of the time. A zombie that runs over to the enemies, vomits acid on them for 3 seconds, and then disintegrates would be a fine example.
This situation actually extends to melee classes too. At some point in progression it will definitely be the case that melee classes cannot keep up as nothing can survive in melee range and only range classes played like a bullet hell game will be capable of playing.