The other movement of change sweeping through developers' design choices is the near removal of classes and/or levels. EVE Online was one of the first MMOs to do away with levels, and the upcoming All Points Bulletin and The Secret World appear to be doing the same. Removing levels makes it much easier for new characters and old character to start playing together right away. Instead of them being level 17 and 58 respectively, one just has a larger skill set than the other.
Then we have classes. Final Fantasy XI moved away from the idea of tying a character down to one or two class types and allowed players to pick any two classes at any time -- wanna be a Warrior/White Mage? Go for it. Now, titles like Champions Online and Fallen Earth are doing away with the idea of a class system altogether. Players can still create an iconic DikuMUD build in if they like, but by allowing them that choice, these games have taken a brave step forwards.
Then again, games like Star Wars: The Old Republic seem to be digging down deep into classes as a way to tell a full blown story. Although I personally think that an MMO is ultimately there to allow people to come together and tell their own stories through the gameplay, so it'll be interesting to see how BioWare's approach will work out. I'm very curious to see how their endgame design turns out.
Whatever happens with MMO design, I'm just happy that technology and creativity are getting to a point where how we define these games is starting to get a little blurry. Blurriness is good, because I love being surprised by a game. I love it when something is unprecedented, just like how the DikuMUD was back in its day. Pretty soon, I hope to be playing the progenitor of a wholly new "traditional" MMO design philosophy.