SL is still best placed of the major virtual worlds to top the market for the over 18's set. Rooms based worlds have restricted applications and tend to appeal more to tweens (always have and always will). I agree with SimGuru that an opensource solution will rule the day in 5 years time, the apache or SQL of virtual worlds so to speak. Consumers may not even be fully aware of the underlying system, as it'll be the solution provider they associate with the world, rather than the technology - (variations of which will be in use by other worlds). In terms of the older crowd I expect the killer world will be associated with the killer app. That could be a commercial collaboration product, or say something in the education market. The application may be narrow, but something for which participation in a virtual world environment has compelling positives.... enough to make it compulsory for the activity, and show significant cost benefits. I'm concentrating on commercial applications here, as in terms of leisure activities, many virtual world applications are better handled by mmogs... and so I expect those activities to migrate to purpose built worlds which are more tightly focused - powerful and flexible like virtual worlds, but really games. Entropia comes to mind as a current incarnation of this type of thing. These applications will probably be born in the rapid prototyping environment of virtual worlds, then migrate (perhaps thru third party providers such as multiverse) into mature social gaming platforms. Because of this I think communication, commercial, cultural, educational, and other "serious" applications, will be the driving force in growing the over 18s crowd within virtual worlds. Now it's just a matter of seeing who serves these applications best - and seeing it's all about user generated content, which communities step up to the plate and does something compelling enough in their virtual world to bring people in from outside.