The culture is so important lately, that it is not only .Net Framework that is getting neglected, also Java. It seems start-ups feel that working with Ruby, Python or PHP is the best way to go for every problem.
But I do believe is cultural. We are building a start-up with several pieces of the service in .Net. And it has been extremely fast for us development wise. We are a team of multidisciplinary developers, and we decided to go with .Net for so many reasons; one of them was speed of development and speed of execution. And it has been great so far.
The weird thing is that we are considering using other technologies for specific problems, like C low level execution, but Ruby hasn't yet been found needed; at least not yet.
We believe that as a developer solving a problem we must choose the best tool for the job, and we believe culturally that is what should be promoted. Ruby, Python, PHP, Asp.Net, C#, Java, C++, Lisp, etc. They are only tools to achieve a goal.
Startups should strive to be problem solvers, not technology evangelists.
Cultural, depending on your desktop platform. .NET doesn't have an official IDE on any platform than Windows—if you run Linux or Mac, it's too fucking bad.
A lot of people who like *nix are precluded from it, without some sort of major workflow change. After getting excited about OSS languages, it's hard to just dump that and go full-stack Microsoft. Hell, a place I just interviewed at literally has everything through Microsoft that's feasible. IIS to the damn mice. And SourceSafe.
Our production environment is running on Windows + IIS + .Net, but we are moving everything to Mono. It is working for us. We are working on a stack based on Debian + NGnix + Mono. We are already using PostgreSql and Mongodb as as a data storage; we use Git (@ Github) for our repositories, and TeamCity for CI.
I do agree, the OSS stack for .Net is not as nice as Microsoft's, but in my case I mostly do everything with Emacs anyways, so it doesn't bother me. MonoDevelop is not bad though.