Or "Why I Hate MMORPGs"
Too slow progression through areas - not so much "i have to stop and grind these enemies or they'll kick my ***" as "if i dont grind them i wont be strong enough for the next area". Especially bad if the game isn't fun initially - "i have to work now to have fun later".
Not enough options in how to attack, which is directly related to how little fun the game is, and there being NO skill curve. This has nothing to do with action vs RPG, by the way.
Almost all of them are third person over the head games, with exceptions being notable - I enjoyed Maple Story a bit just because it was a platformer, but utltimately the ridicolous grind got the better of me. PlanetSide was cool but had some seriously frustrating elements (GOD DAMN ###### PHOENIX ROCKET LAUNCHERS AAAAAAAAAAAAAAAAA), plus it pretty much died. Third person over the head works for Zelda because it's a singleplayer game, basically - story and lack of lag and such things.
Everyone wants to be special, so there's a lot of variety. Unfortunately this means less quality, more quantity... Abilities are very very often less satisfying than they should be - anyone who has used the Fireball spell in Guild Wars will know what I mean. One of the best attack spells in many ways, it just produces a small flare-y effect and a 2D ring of fire sprite on the ground. Ugh.
Starting from absolute zero. You hit Giant Rat for 2 damage. The Giant Rat hits you for 3 damage. ... You drink a health potion and gain 30 health. The Giant Rat misses. ... You miss the Giant Rat. The Giant Rat misses. You slay the Giant Rat! .. fortunately we're mostly past this. Even random missing is becoming less common, or at least less frequent. In City of Heroes there was nothing quite like pumping a Stalker up with every short-term buff, starting an Assassin Strike, only for it to miss and all spent effort to drain into the sand. Like it wasn't a weak enough archetype to begin with...
Music. I know not everyone can be CAPCOM Sound Team, but either the music sucks or it stops after 20 seconds (city of heroes).
Potions. I have 5 more health potions in stock than my opponent, therefore I win.
Uninvolving characters and story. I have no idea why it works better in singleplayer games, maybe it's just some bit in the back of my head that always knows that tons of people are watching or have watched this major event of a town being destroyed or my character being knocked unconscious or whatever before, while simultaneously being in the same world as those who haven't.
Tons of little things that vary a lot with the games, like having to repair or not being able to do so without a long trip to some city.
My biggest gripe is with the gameplay. If every encounter is the same, then there's no reason for me to keep playing. This ties in a lot with slow progression; if the level design changed according to enemies, and their tactics to the level, that'd still keep things interesting - see singleplayer games.
Over the head perspective might be what really ruins it - walls and ceilings have some serious restrictions on them when that view is used, you have to allow people to select their targets with ranged attacks or hitting will be very difficult, obstructions can be easily seen past rather than have to be carefully navigated, surprise attacks are never very surprising..
More follows as I come to think of it.
TL;DR MMOs should stop trying to be singleplayer games EXCEPT ONLINE!! and start being massive worlds with the player as the important inhabitants.