Eto.Forms is an abstraction layer on top of WPF, WinForms, GTK, and MonoMac/Cocoa - so you can get a native UI on all platforms without having to re-implement for each platform. You don't have to suffer from lowest common denominator, since you can implement specifics for each platform (if desired, but not required).

It has an advantage that it is pure .NET and only relies on OS-supplied dependencies, as opposed to having to bundle native binaries for various platforms with other toolkits.