Woah, wait... three factions?! That's so fresh!
Now, don't get me wrong, TSW is a decent game, and it did at least attempt to do something new with the tired old skill trees. But still, come on, you have to bite your tongue pretty damn hard if you want to pretend that it's not a WoW-clone at heart. The core gameplay is lifted directly from the first page of "MMO 101: So You Want to Craft War in the World?", and so is the user interface. It looks like WoW. It feels like WoW. It acts like WoW. Granted, it's the WoW that Cthulhu would play, but it's still pretty much WoW, minus the financial support and craftsmanship of Activision/Blizzard, and that's not a good position to be in if you want to take on the WoW that has the financial support and craftsmanship of Activision/Blizzard.
So is that it? Is that the reason MMOs that aren't WoW are dying like a bunch of flies repeatedly banging their heads against a closed window? That they're trying to fight WoW on the terms set up by Blizzard? Yes and no. The way I see it, there are two main reasons they're all failing. First of all we have the absurd notion that MMO makers carry with them, the one that makes them think that they either have to surpass WoW in popularity or pack it up and go home. But then we have the second reason, and this is the one I think is truly important: failure to identify and target the right player base.