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For me it is any sense of depth or complexity, combined with a lack of community. In fact I would summarise it as follows: -
1. Game play has become simplified, and twitch does not make a game harder, it just requires more attention than I can give to an MMO which requires many hours of game play. I have other things I need to be doing.
2. Mechanics are solo-centric; you sit in a hub waiting for an event to pop. All other areas of the map are obsolete once you have passed that level.
3. Levels persist, but in a diluted manner, you can alter your level on a whim or assist others; all sorts of mechanics have been introduced that make levels entirely pointless; yet no-one has had the balls to remove them entirely.
4. I know you should make your own social experience; but this is only part of the story. A game needs to give you a reason to socialise and most modern games do precisely the opposite.
5. The same old tired questing mechanics are rolled out time and time again but given new names (GW2 is a prime example) to try to hide the utter tedium involved in repeating the same old content we have been playing through for years.
6. I could go on...
So what is missing in new MMO's for you?