I think what is need to make MMO better is one of two things. Either they improve enemy AI a ton or they start paying people to be the enemy on the other end.
If I am told in a game to go kill 4 of the same creatures then it gets really boring, not because I have to go kill 4 of the same creature, but because I know that the fight will be the same every time. Find creature, who almost always is standing at the same spot , sometimes looking like a stick figure glued in place. Kill creature , spamming it with the same tactic, because it always attacks the same way. Return for reward. If the creature I was sent to kill actually did things like a real creature would I wouldn't mind it. Let it be interacting with other creatures nearby. If it is a camp of orcs then let them be cooking dinner, talking to each other, fighting, something besides standing in a spawn spot. If I attack something then it needs to be just as creative as I am in trying to kill it. Not just stand in one spot while I attack . Let it cast a spell randomly that it only cast once in a hundred times. Maybe that creature knew something the others didn't ?
If I kill something carrying a sword it better have a sword on it when I loot the corpse. If I am attacking something like a cow and a wolf wanders by the wolf should come to watch like a real wolf would. It may not join the fight, but it should investigate what the noise is. It shouldn't run over and start attacking me while I am fighting as a real wolf wouldn't do that.
The problem with MMO is that the worlds are filled with NPC that are cookie cutter creations. Same scripted responses and actions. If I am playing a game for hours and hours each week then I don't want to see the same guy , standing in the same spot, doing the same thing, every single time I enter an area. You might as well put up a picture with icons to click and save development time.
The other problem with MMO is the mentality that it should be like a single player game in that everyone gets to experience the same content. Sorry that just ruins it for everyone. If there is a boss monster and someone kills it that should be it, no more boss to kill again. People want immersion and a game with a story they can get into, but at the same time they want everyone to be able to kill the same monster over and over. Warhammer ruined it for me when they said that cities would be reset. That kills the immersion. If players want something back they should have to take it. If a monster dies then let it be gone and move something else in place of it. So you killed the boss monster in his big castle. Well, you didn't keep enough of a player presence there, they all left because there was nothing to kill, so the monsters from another part of the game decided to make it home.
What an MMO needs is to stop thinking that the players will be the ones that make the content come alive. They need to create worlds that function without the player then drop the player into it.