For me I would go ahead and say Scenarios. They look great on paper, but have been poorly implemented so far. In my opinion for Scenarios to work, they need to be used for the levelling process, they have been called queued group quests, however group quests are really only good to grant experience.
Many scenarios scattered throughout the levelling process would greatly improve this feature. Current scenarios could open up per level from 85-90, lower level scenarios wouldn't scale (similarly to dungeons).
It wouldn't take long to queue for lower level scenarios as well as a levelling toon, with the fact you can queue to multiple things at once now.
So, MMO-Champion readers, what is the feature that has been the most disappointing for you in MoP so far?