Going off my short burst play tangent, last night I had another great experience in Wizard 101. Below the Krokosphinx there is a hub that leads to four other areas. However, in that area is a boss named Grumish Greataxe. Grumish can only be summoned by players that have completed a quest chain, but once summoned anybody can join.
I signed on excited to get to the third and final hub in Kroktopia, but people were fighting Grumish. So, I joined them because joining a boss fight can only make things better for all. We beat him pretty good, and I got to flex my mighty-Potter pythons with lightning versus the myth boss (lightning beats myth, fire beats ice, etc.). During the time we smacked the minotaur around a small crowd had gathered, and people in the crowd were rearing to take the boss on as well. So, I joined again. Then again. I never made it to the next hub.
It was just a blast to fight this boss with a rotating group of other people knowing that I could leave at anytime between fights without anyone batting an eye. While there is good and bad to the transient nature of grouping in Wizard 101, the emergence of this pick-up style play will rarely happen in a more traditional MMO. I think the only other time I was this excited to just play with people at a whim was in the early days of Warhammer Online where public quests were actually populated.
I hope that true pick-up play will occur more often in the MMOs of the future.