Davy Brion, a .NET developer, writes about software craftsmanship, career advice, and the trade-offs between languages and platforms like Silverlight and HTML5.
HN links it in discussions of .NET versus startup culture and in a point about compiler and interpreter warnings being useful learning opportunities.
I'm sure if you Google around you will fine a number of opinions on why the majority of startups don't go with .NET, but I'd say it is possible it is simply because many .NET developers love in the 'enterprise' world and are far away from the startup world most of the time. As a .NET developer I'm seeing this starting to change but it is only just starting.
There are certainly some disadvantages to .NET particularly when developing for the web, namely it's web framework maturity, ease-of-use and the lack of a strong open source community. These things are often cited as reasons why, say rails developers, are able to go "faster". And by faster we are not talking about performance but time to build your product. I'd say I find the last one, lack of a strong community as a big factor.
Anyways, I probably can't help you answer this personally, but here are some recent posts by fairly prolific .NET bloggers on why they decided to start moving away from .NET for what they feel are greener pastures in the rails community.
http://davybrion.com/blog/2010/08/how-i-feel-about-the-net-world-lately/
http://wekeroad.com/post/7468824379/i-find-your-lack-of-faith-invigorating
http://osherove.com/blog/2011/1/2/the-journey-begins-and-why-it-starts-with-ruby.html
And a specific article about Rob Connery moving his startup from .NET MVC to Rails and why
http://www.infoq.com/articles/architecting-tekpub
Again I don't mean to advocate rails over .NET, but I wanted to answer your question…
ruby warnings work pretty well
http://davybrion.com/blog/2010/08/compilerinterpreter-warnings-are-important-learning-opportunities/