As I understand it, windows are responsible for drawing their own window decorations on Windows and OS X, but on X they're drawn by the window manager. So it's harder to mess with the title bar because you're starting from nothing and don't even know which of a dozen decoration engines is actually being used. Upside is that decorations are remarkably consistent under X.

Wayland matches the Windows/OS X behavior (probably because it's easier to just accelerate one big rectangular canvas the process draws on), so who knows what will happen as that catches on.