My personal excursion into Raph Koster's Metaplace didn't last very long. I felt very disappointed, because what I had expected was a system to build virtual world games; it turned out that Metaplace is a system to build virtual worlds with little or no game functionality. Sorry, I'm about 40 years too old for playing in sandboxes.
So as I'm not going to cover Metaplace on this blog, I'll link you CNET's review of Metaplace instead. Just be aware that Metaplace is a "virtual social place", and doesn't really have anything to do with MMORPGs.