The ability to express your schema in GraphQL instead of JS is especially nice. The server+client Apollo experience is really nice if you give it a try, and judging by the roadmap laid out in the blog post there are a lot of benefits to come.
Windows Forms is only a subset of the framework. I don't think exposing Windows Forms as an open source GUI stack would be worth while. It's old and clunky compared to WPF and XAML.
Even without a open source GUI stack the open source .Net Runtime and Framework provide value to .Net developers like myself who enjoy C# (and the F# lovers too!) to develop on Linux, FreeBSD, and OS X with much less pain than in the past. Not fitting each and every scenario doesn't diminish its significance.
http://dev.apollodata.com/tools/
The ability to express your schema in GraphQL instead of JS is especially nice. The server+client Apollo experience is really nice if you give it a try, and judging by the roadmap laid out in the blog post there are a lot of benefits to come.