IMHO - these days MMOs require a massive amount of time, energy and money to make. Most, if not all, developers cannot gamble and create something unique and original in fear that it may fail, so, we have a landscape where developers create an MMO with a tweak or two and claim to be breaking the mold; i.e. SWTOR, RIFT, GW2, TERA, etc., each game claims to be breaking new ground, but they're only offering a slight tweak on the tried and true formula. The same can be seen in single-player games where each year we're bombarded with sequels, again, its less risky then developing a new IP with no known fan base.
I think if we're going to see a fresh and new MMO it won't come from a mega-developer, it will come from a small start-up, or one person, think Minecraft.
So, you couple the fact that developers do not want to take big risks add in a jaded MMO community and you have what we have today = stale games.