I still wouldn't run my startup on .NET though, for a number of reasons. Mono simply isn't there yet. Code that works fine on Windows mysteriously stops working on Mono due to threading or GC vagaries (yes, even in the latest version). This isn't a dealbreaker, but it is pretty annoying especially since debugging these kinds of problems are of precisely the kind that I don't want to debug.
The ecosystem is really quite poor compared to, say, JVM or Node. For all the faults of either, there is a LOT of stuff going on and there is a LOT of innovation that is sorely lacking from the .NET world. .NET usually catches up eventually but it's either late or half-hearted (just look at the progress of NHibernate vs. Hibernate).
I think ASP.NET vNext is a step in a very good direction and I hope to be able to change my opinion sometime next year.