These are purely my first thoughts - I haven't put any research into fixing issues yet.
The biggest things that annoy me about the OS:
1) Difficulty in switching default apps - it's designed purposefully to keep Windows 10 users on Edge and Bing.
2) Windows 10 - the anti-privacy edition. Make sure to check your privacy settings if you don't want more data than Facebook has on you reported back to Microsoft...
3) Can't wait to see ads on Windows 10/future iterations of the OS. :r1: