I'm using a tiling window manager with mostly GTK apps, so pretty much all menus and such look the same. The worst offenders are Firefox and IntelliJ, although they have improved a bit lately.

However, I'm not sure that this is the reason for lack of adoption. Windows has always been a patchwork of interface design, every other app having their own special window decorations and behavior, including MS' own apps (thinking of Office in particular). Also, seemingly similar elements, such as text inputs, behave differently in different places. For example ctrl-backspace sometimes deletes the previous word, sometimes it introduces random characters.