Shortcuts doesn't work (for example [Win]+[Down] should restore the window when it's maximized), the window can't be dragged from maximized, the window doesn't maximize when dragging it to the top of the screen. Furthermore the window doesn't animate when minimizing or restoring from minimized.
It's been like that for as long as I've been using Spotify. They don't stick to any Windows standards, and the application has painting issues like no other windows application I've ever seen. I always assumed it was some funky development engine that was there to improve speed, but the latest updates to Spotify would betray that theory.