I work in a startup that chose .NET for our product and admittedly we only did it because one of our developers had a lot of experience with .NET. I definitely wouldn't use .NET a second time, mostly because as a startup I value open tools and freedom. I started working in startups to get away from corporate bureaucracy. And now with hindsight I now know that "just because your dev has experience" is never a good idea to do anything.
On that note - I'm working with PHP's Laravel framework for my next project, and despite the performance tradeoff it's an absolute dream to work with.