Too many people obsess about backends. Yes, build your startup in C# and a .NET stack if that's what your familiar with. The backend ultimately doesn't matter, and the faster you can create and then iterate on your product, the better off you'll be. The primary factor that will decide whether your startup is successful or not is if people like your product and actually use it. None of your users will know or care what you built it in, be it .NET, RoR, Django, Yii, whatever.
The other person in my head is saying. "OH MY GOD NO". I've used the .NET stack and various OSS stacks to build complicated web applications. From this experience, I've concluded that the .NET stack sucks. Yes it works, but almost any OSS Framework will provide a much better developer experience. Better while developing the initial product, and better when maintaining and extending it.