Bob Moore from Terra Nova has written a nice summary of talks by WoW lead designer Rob Pardo and Bioware lead combat designer Damion Schubert at the Austin Game Conference, analysing the success of World of Warcraft. I've got a post in progress about Mr. Pardo's talk in particular, but hey, who knows when I'll finish that. This is a good summary.
Even though I occasionally stop playing for a bit, I am still involved in World of Warcraft, over a year and a half after it came out. It continues to offer new experiences, and I am sure I won't have exhausted the content before the expansion hits the stores. I continue to be impressed by how well-tuned the game is. As someone pointed out to me once, WoW is the first MMO developed by successful single-player game developers, and it shows.
This report by DFC Intelligence points out that WoW's success will be extremely hard to replicate, and that a lot of people are going to try, due to an influx of dumb money. I find it much more interesting to watch different MMO models.