A write-up at Gamasutra of an AGDC panel featuring MMO developers who have begun working on web-based projects gives us an opportunity to explore the new (some would say it's actually the old) frontier of massively multiplayer gaming.
There are many MMOs that are experienced via a web interface (such as Sherwood), but traditional gamers have largely shunned the trend. Why, then, did some of the most hardcore MMO developers (including Dan Ogles, Raph Koster, and Scott Hartsman) abandon the traditional MMO in favor of this new frontier? Some of their work, like Ogles' Loudcrowd, is barely recognizable to traditional gamers.
They offered some answers on the panel. For example, Koster (originally of Ultima Online and Star Wars Galaxies fame, now working on the creative platform MetaPlace) said that game devs have more to learn from web devs than vice versa. Ogles talked about using Adobe Flash so anyone can embed elements of the game anywhere on the web they like. It's worth a read if you're able to work through some pretty technical development speech.