Though I don't really consider myself a .NET developer during my career I did manage to get involved with a few of these folks. I was lucky to meet some real experts of this field yet majority of the time I was surprised to see how many people actually call themselves .NET developer - or filling in such role - without knowing even the fundamentals of it.
In short, I already gained significant exposure to the following .NET-based goodies:
- XBAP/WPF development - created a multi-threaded server load test tool
- ASP.NET - mainly web-forms...many of these
- IIS configurations - URL rewriting and basic configurations
- WinRT and Windows Store 8.1
- WinRT and Windows Phone 8.1
- Office Web Add-in (just touched upon)
- Visual Studio 2013-2015 and Visual Studio online - plus source control management
That's pretty much it...