[billyhoffman]

My use case is the most basic one possible:

-Build a GUI app in C# -Run in on both my Windows dev machine (Windows 7), and my main desktop (OS X)

That's it. I don't care even what I have to use to build the app (Windows Forms, Stetic, etc).

Any advice?

[visualR]

If your app is something others will use, for the best experience you write your app in such a way that the UI is as swappable as possible. Then write a Mac UI with monomac and a Windows UI with WPF. If it's just you who needs it, then Gtk# might be ok for standard things. But don't expect good os integration without work.

[jonpaul]

Have you considered using node-webkit, AngularJs, and Bootstrap for your cross platform GUI needs?

[visualR]

The problem with this is you only get 1 thread for the UI and other stuff to run on.