Nothing makes a game feel less like an MMO than having to run around its world for two or three hours and seeing almost nobody else playing alongside you. Having nobody else to interact with, aside from NPCs who dole out objectives, is not why any of us are here.
This topic is nearly a column all on its own (and is a long-standing discussion amongst the MMO community) because nothing is better than playing with good friends but being on your own is nice, too. You can get bored of both, and so pursuing solo and group play is worthwhile. And as nice as it would be to assume all developers can create a good environment for both, that's not true.
While I don't think MMOs have "lost their direction" with solo content, I do think something ineffable is lost when a game focuses too much on solo or group play. For a developer to fill an MMO with equal parts solo and group is no easy feat. We're demanding two games here, and to expect as much is probably unreasonable in almost every situation. Sure, BioWare has some serious funding for Star Wars: The Old Republic but that doesn't mean they have enough -- or that so to do other developers.
For better or worse, everybody's tastes differ. The trick is to find a game you and some friends all really enjoy as it is, without any personal caveats. This, coupled with taking a decent break, will surprisingly revive much of that long lost enthusiasm for playing an MMO for long enough to reach end-game, develop several in-game relationships and everything else that comes with the complete MMO experience.
The fact of the matter is that, given enough repetition in a measured period of time, anything can (and will) become boring. But even though a lot of differences come into play when you ask someone why they tire of an MMO in "X" amount of time, everyone shares the entropy factor. That is to say, everyone has a limitation to how much of any one game -- or any one genre -- they can play and have fun while doing so.
There's no way for a developer to stop this, because it's human nature. Hell, I'm sure they take occasional breaks from their own game simply for their own sanity. I wouldn't be in the least surprised to find out that several videogame developers go through an extreme version of this cycle of boredom followed by an almost zealous return to the MMO gaming space. Perhaps this is the real reason the MMO genre has been slower to evolve than other videogame genres.
As they say, the best medicine is rest. So if there's an upcoming MMO you really want to be good, perhaps the most effective method of ensuring your enjoyment would be to simply do something else while you wait. I heard Mass Effect 2 was pretty great. I also recommend the books Spin and Name of the Wind.