The way a player levels in WOW has changed significantly with the introduction of the dungeon finder. For those who do not know, the dungeon finder allows a player to flag themself as lfg and allows them to choose a role (DPS, Healer, Tank). The dungeon finder then looks at players on multiple servers and creates a group for an appropriate level dungeon. It then transports these players to an instanced dungeon.
This has turned leveling into a multiple player experience again. WOW was once known for making a questing system that allowed solo play fun and accessible but now things are changed. I don't think players are looking to run so many solo quests (been there done that). So the content that an MMO designer has to create has changed. There is still probably a demand for solo content but it is more likely something people will want to do when they are waiting for a dungeon. So they may be level 20 when they get a quest but they may level in dungeons to the point the quest is no longer needed.
When looking at the current design of WOW it is clear the game was not designed with the dungeon finder in place. So the question becomes how do you design an MMO assuming you have a dungeon finder system in place? How can you make it so that the world still matters and the game is not about going from instance to instance?
I think the old(WOW) way of designing worlds can be thrown out. I think that designers should create single player quest chains that are independent of level which are complimented by multiple player dungeon crawls. I think using a side kick like system can allow for a lot of improvements in this area as well as sliding scale difficulty for dungeons. I think that players can be teleported to specific areas in the world so that the dungeon crawl can exist within the game world and not just in an instance.
There are a lot of options here.