Complete lack of any AI for the monsters

[Walkiry]

But how playable is a game where any MOB goes first to kill the priests, mages, and warlocks, druids, and then rogues before turning on the lower DPS turtle toons? 
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Why any mob? (By "any" I think you mean "every", correct me if I'm wrong). Let's say you're fighting Majordomo, the tank is at the top of the hate list as per "standard" combat. Suddenly, a small, easy to kill, not so dangerous fire imp pops out of the lava and goes straight to the cloth wearers (he's in a lava pit, right?). Then another. And another. Soon you have to have a portion of your raid focusing on these easy to kill enemies to keep the back line intact, or else they'll end up swarming you. You don't even have to have these guys constantly out, maybe only for a crucial point of the fight, and if you get Majordomo below X hp they despawn (so you can kill them, or just snare them long enough to reach that point, whatever).

Even if it's not a very good example, the idea is not to have all the enemies going after the clothers, it's to give some a different set of "rules", rules that require something other than the standard hate list mechanics to make alternative methods for dealing with enemies more interesting and rewarding.

Say, a very tough end boss that can be weakened if a rogue picks the lock of a chest in the back of the room and uses a magical artifact on him. When the lockpicking starts, the rogue goes straight to the top of the hate list, and the team has to slow or stop the boss before he tears the poor leather wearer to ribbons, just long enough to get that artifact. Or, you can just smack him silly with your standard tank if your team composition is more suited for it. Or instead, have the boss run to a machine when he's low on health to activate a Goblin Engineering mega-mech (build by a kidnapped goblin, for example, that'S a quest right there); you can try to kill him before he reaches the machine, destroy the machine if you have more appropiate damage for that, or if you have engineers you can have them destroy only parts of the machine and salvage items that can be used to alter the composition of the mega-mech, making it easier... or harder, as any good Gnomish/Goblin Engineering Device should be ;-) Defenses pop out to protect the machine, so it's not trivial to just alter the mech; or just destroy the mech and be done with it, as any highly defensive tank would prefer.

In short, give more than one way to peel the same banana!

This really now descends into game theory.

And, thus, getting more interesting ^_^