1. Learn how to work with XAML
(1.5. Know how to make good looking UIs)
2. Done.
Really. If you're forcing yourself to use WinForms, you're doing yourself a disservice. Any Windows Dev that wants to push out good looking application should learn how to use XAML. You can even ignore the databinding part if you want.
That said, TextNut looks an awful lot like OneNote UI wise.
I believe the major problem comes from the fact that most people developing for Windows are either doing it for free or for internal tools. There are very few tools that fit Apple's market of consumer focused applications, and the financial incentive clearly isn't there on Windows.