I personally love the idea of episodic content in an MMO environment. Right from the start you’d design the game so that it would be fairly quick and easy to add new content on a weekly basis. It wouldn’t be huge amounts of content, but it’d be enough to tide people over until next thursday.
The game would have one huge story arc that would cover the the entire season and beyond. Each episode contains content that would drive it a little bit closer to that end. Within that season there would be smaller arcs that would last a few episodes and plenty that would last only one. Some of the content that drives the story becomes unplayable as the next episode is released (with a day or two grace period), but much of the content would be fairly independent from the main story, thus allowing play anytime.
New players start in the current episode.
There are no levels. There are skills, some gear… Some skills and gear are fairly rare (only a small handful of players could have it), but if you don’t use them, then you lose them. Alot of this gear and skill would be a part of the story, thus the turnover.

In real life, individuals are always changing and experiencing new things, yet we generally don’t feel cheated if we found out we missed something about that person or in that person’s life because we met the person when he or she was 40 and not 30. To the contrary, a lot of entertaining conversations involved the sharing of stories about things we missed…not just stories like “once, I…” but also “once, old John here…”. It’s common to talk about the experiences of friends and family, and not just ourselves.

It would be great if players shared such stories about changes in the lives of their favorite NPCs. In this way, one player might actually feel “closer” to a particular NPC than another player does, and might feel closer to one NPC than to another…thereby creating an illusion of a personally-relative community involving NPCs as well as players.

There also isn’t an immediate need to invalidate old content when done carefully, so the older story arcs could still be made available- giving people who ARE interested in more than a 1-night-a-week thing something to do:

In City of Heroes-talk:
- Your contact gives you rumor of a new villain in town. You investigate, running through a half-dozen mission maps, cutscenes, etc, pretty much a complete story. In the end, your contact’s gotta do some more footwork…
- Next week, new episode’s available. Now, your contact’s got some more leads- that bad guy you took out last episode (or… if you missed it & don’t want to go back… some bloke that just got arrested) had a bit of a network smuggling goods into the city- and they’re moving something big!
- The next week, a character development piece..
- And so on…

Finally, that arc wraps up. The next episode is initiated by a different contact.

If you followed along every week, you have the whole story as it went. If you missed a week or came in late, the contact can start you on the older content OR just let you jump in mid-stream- your choice… heck, if you really liked the content, maybe there’s a “retcon” mode that lets you replay older episodes at your leasure.

The old content can stay up as long as it doesn’t conflict with the continuity of another adventure- say… when your previous contact is murdered.

The idea would be to offer bite sized (Moorgard used “chewable content”) bits of story on a frequent schedule and really cater to the people interested in THAT KIND of content