There's been a lot of discussion around SL lately, for me it's still a fantastic example of how we'd might experience the web in the near future. To get a better understanding of how to get to that future, I recommend you read this interview with Ogoglio's founder. Many of the challenges facing the 3D web are being discussed, including some of the flaws SL's model (and also, at least for now, Croquet's).
“Sorry, but Cory was right. Croquet and MUPPETs are both trying to beat Second Life by being a better Second Life, and that’s a game that’s already lost. We don’t need a better stand alone 3D client and server grid that barely talks to the web. We need the web to get better.”