Like it or not, but there is a definite stigma associated towards working on Windows. Developers on HN are surprisingly open-minded towards developing on Windows, but in the real world you'll find a lot of developers that look down on those that choose Windows as their development platform.
I've met a range of devs, from those working at startups, to people who work at Google, and many of them have been extremely dismissive of not just .NET, but those that work on Windows. I've worked at a startup using ASP.NET, but I reckon many C# devs would probably rate their chances of getting a job at a big tech company to be slim and none because they favour .NET. I spoke to some ex-Googlers a while back at an event, and I asked about the Google interview process. They asked me what language I coded in, and on hearing that my main language was C# they were less than favourable about my chances. It's a shame really, because I'd quite like to work somewhere like Google, but I wouldn't want to have to hide the fact that my strongest language by far is C#.
In my mind, bringing .NET to Mac and Linux will do something that we rarely see in the development world. The Windows world will be bringing something valuable to the Linux world. The best part about this for me isn't being able to use .NET on an OS that I'd love to learn more about, but not having that niggling doubt in my head about my choice of language/framework, and I reckon a lot of .NET developers, especially younger ones will feel exactly the same.