I think this story is so interesting because so many of us could never imagine leaving .net. It's got a real shock factor to it. Like someone winning the lottery and throwing away the ticket.
I also had a bad experience with mono. I tried to run a very heavy site on it when asp.net first became available for it. It totally crashed. I'm sure it's matured a lot since then.
After that happened I saw a blog post by someone at microsoft about their testing. They have 1.6 testers for each dev working on asp.net and they've got a warehouse of servers running millions of different tests for months before shipping.
Then I read about who was working on mono's asp.net implementation. Just two guys in a basement. No testers of course.
I think that the recently announced NuGet (http://nuget.codeplex.com/) will be a big boost to the .net community. Having the global library repository like Perl's CPAN will be huge in bringing the community together. More users of libraries will mean more improvements too them.
It also seems like the existence and success of sites like codeplex and stackoverflow negates his thesis.
Was glad to read that he was leaving for purely non-technical reasons. I bet he'll be back someday.