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One of the things I remember fondly from vanilla WOW was the epic class item quests. You know, the Priest quest chain to get Benediction or the Hunter quest chain to get Rhok'delar.
OK, you did need raid-drop items for these, so you had to raid at least a bit (and probably more than a bit), but the really memorable parts to me were the exclusively solo steps (where you could get no outside help). Like the final heal-a-thon in Eastern Plaguelands (for the Priest) or defeating the four demons in different zones (for the Hunter).
These quests gave you some solid long-term goals to chase and prep yourself for, and some exciting encounters. They provided fulfillment not only through the special items themselves, but through knowing you've completed a series of difficult-ish steps successfully.
I thought having such solo-oriented high tier PVE challenges along with the group-oriented high tier PVE challenges (i.e. raiding) was a good match for a directed-content game. It gave you something exciting to work for when you were on your own and the reward was actually good. You also needed to buff yourself as well as you can for these (because you have to be the best you can be), and sometimes seek some strategic help (though not direct), so interestingly, it actually made you go and interact with the community, too.
Now, don't get me wrong, I like having group-dependency in my MMOs. And a lot of it. And vanilla WOW did have that. But I felt having such "special" solo goals on the side really added something to the game.
So what say you? Epic solo quests - do they work in MMOs?