There are many divisions among the MMO audience. Casuals versus Hardcore. PvP versus PvE. Gevlon's Morons & Slackers versus all the non-M&S. Roleplayers versus Everyone Else.
But there is one conflict that I think is more important than the rest. The conflict that essentially shapes all the contortions that endgame goes through. This is the conflict between Transient and Extended content.
Transient group content is content that is expected to be completed in a single session of play. The group is formed, the group completes the content, and then the group is disbanded. In WoW, group quests, battlegrounds, and 5-man dungeons are transient group content.
Extended group content is content that is expected to be completed over several sessions of play, and where the group is composed (more or less) of the same individuals throughout. In WoW, raids and PvP arena are extended group content.
The MMO community is divided into two factions. Those who are willing to do Extended content, and those who are not willing, who only want to do Transient content.
Please note that this is not about skill, or anything like that. It's about time, and the willingness to follow a schedule, or accept a constraint on your free time. It's about being willing to undertake obligations to other players. Many people simply do not want to be obligated to other players, or for other players to have a duty towards them.
It's also not about the amount of time played. There are Extended players who play once a week for two hours. There are Transient people who play for 40 hours a week. They play so much that it is really hard for Extended players to understand why they will not commit to a schedule. The stumbling block is the existence of the schedule at all, not about the total amount of time played.
In the past, it was assumed that people would move from Transient into Extended. That it was a natural graduation. You joined a guild, and started working with that guild on Extended activities. But I think this was a bad assumption. Rather, the Transient people are simply different from Extended people. You can't change them into Extended people. You can go the other way, however, in that Extended people will do Transient content.
Traditionally in MMOs, Extended players have made the core of the playerbase. The raiders, the high-end PvP players, the stable guilds. But I think the Transient players actually outnumber the Extended players significantly. And the Transient players want the game shaped to meet their needs, not the needs of the Extended players.
As an example of the design choices imposed by this conflict, consider the Guild Perks system that was introduced in Cataclysm. I don't think this system really made any difference to the player base, and to be honest, was probably a waste of developer time. The Extended players already were in guilds. The Transient players simply joined uber-large guilds to get the perks. But it did not change the nature of the players.
The single biggest problem with the endgame of WoW is that it persists in believing that if the incentives are just right, Transient players will transform into Extended players, and everything will work out properly. But this transformation never occurs. The Transient players are unhappy that their needs go unmet or that they are thrown one or two sops while the Extended players get all the goodies. The Extended players are unhappy because those sops for the Transient player often end up distorting the Extended endgame.