The 2 Real Reasons Second Life is Successful
People talk about why Second Life is successful. Sure it was timing and was one of the first big players. Sure they had awesome news coverage and kick ass marketing. Sure they offer some of the best features and graphics out of the massively multiplayer online games out there. But the sucess really boils down to two points:

1. It is free. There is no getting past it. They were the first major free MMO when World of Warcraft was striking it rich off of the pay to play model. Say what you want about Second Life reliability, etc but hey it is free and you get a lot for what you are paying for.
2. The users retain the rights to what they create. So many other MMOs give users the ability to create but try to maintain ownership. People do not want this. This is why their virtual economies struggle.

SL is also based on a philosophy that adults can manage themselves. I refer to the emphasis on Tolerance and even appreciation of diversity, and I think it is fundamental to most residents’ enjotment and sense of ownership of their Second Lives. To quote the Community standards themselves: “The goals of the Community Standards are simple: treat each other with respect and without harassment, adhere to local standards as indicated by simulator ratings, and refrain from any hate activity which slurs a real-world individual or real-world community.” The prohibited behaviors include acts of: Intolerance, Harrassment, Assault, Disclosure, Indecency, and Disturbing the Peace. http://secondlife.com/corporate/cs.php
Adults want to and are capable of governing their own behavior; aside from a few morality fundamentalists, they don’t need or want a corporation or government imposing behavioral standards beyond freedom from Harrassment. Any virtual world not firmly grounded in understanding this is unlikely to remain interesting to adults.