Helistar encouraged me to blog on this:
Several MMOs have been criticized because they turn into "two games packaged at one", i.e. the leveling game (quest-based, mostly solo) and the endgame activities (raiding/pvp), which don't share much in common.
Helistar wrote more than that. If you're interested, go check out his comment. On the above quote: I agree that this has happened and I agree that it is not optimal.
Let's first try to understand how it happened. MMORPGs were typically about the 'leveling game'. The designers didn't think about the endgame all that much. They didn't have much hope to keep people in the game after completing the leveling game.
However, it turned out that just attaching some PvP/PvE minigames (endgame) made players play the MMORPG for years! Woah! Nobody expected that. With WoW Blizzard focused more and more on that endgame, because it was so extraordinarily cheap to produce and kept millions of players p(l)aying.
Now, this is a working model. That's why a lot of companies clone it. The leveling game lures players into the game and the endgame keeps them paying for a game they might never have bought, if there hadn't been a leveling game.
The reason there are no pure raiding games is that raiding isn't all that interesting, really. Neither are most endgame activities. These activities are interesting mostly, because they feed on the meaning induced by the leveling game, the community and the simulation (illusion, really) of a world outside the instance.
Now, after some decade of playing these games, many gamers are bored of it. What can be done about it?
Well, if the single goal of a game is ever increasing character power, not much can be done about it. You can try to find some way to get infinite content. Like Psychochild's storybricks. But if the only goal is to increase character power, all games become boring eventually. And this does not only apply to the specific game, but all games that have character power as goal! Once you played one of them, you played them all.
Humans may be a bit stupid, but they aren't complete morons. We understand quite well that something is wrong when we hit level 77; and it doesn't feel as fun as level 5. This is especially true if all our opponents level with us.
My favorite way to solve the problem is to shift the goal of the game from character power progression (CPP), to other things. For example to defending against an ever-ongoing (and seriously dangerous) enemy undead invasion in a PvE sandbox. You can still keep a weak CPP, but you don't run out of content, if the content is created by the interaction of the players instead of an external designer.
Moreover, the goal of the game is now sustainable indefinitely. There is endless content, because it is created by the inter-player and inter-player-group relations. And we know from the real world that this is interesting for .. forever.