This is going to be BIG! - 10 Reasons to Go Short on Second Life worth a read to see not only Charlie's reasoning, but also to see the comments from others that question his reasoning. It is good that some questions are being raised as Second Life continues to receive extensive news coverage - I've seen three national news items on television and heard two radio news presentations on Second Life in the last 4 weeks.
I am skeptical about Second Life - primarily for the reasons cited by Stephen Downes - it is not democratic, it is owned by a single company, and that has a lot to say about what freedoms are accorded to the online user. I'm not interested in investing a lot of time and energy into creating perosnl or educational aplications that are "owned" by someone else, and could be used for purposes I did not design them for.
As a visual medium it is attractive - but not terribly innovative. It is just creating the saem structures we have in real life - it's educational uses are great for say the behavioural sciences. I am also apprehensive about the continued impositon of corporate interests into Second Life - product placements, sponsorship, advertisements, billboards, soon enough it'll be come the virtual mall linked into paypal and catalog databases.
Open croquet is an educational immersive environment that might be more to my liking - if I were more savvy about programming.