If you look at today's mainstream online games, it seems as if that art has been lost. There are some smaller communities out there that still herald that kind of experience, though. The Escapist focused on one of those in an article titled "The Ending Has Not Yet Been Written." It's an excellent piece about how players and developers alike have fostered a unique, niche-storytelling experience in Cyan Worlds' Myst Online: Uru Live.
Myst has always been an eccentric in the gaming world. It was a groundbreaking success for computer games, and it spawned countless clones, but no one ever recaptured its magic. Now the series is treading a unique path in the online world, despite past setbacks.