Unity's implementation of a global menu is probably the most consistently functional I've seen at the very least. KDE has this feature but compatibility is more spotty, partially thanks to necessary packages going unmaintained and on my main distro (Fedora) missing altogether.

I agree.

Xfce has a plugin to do it as well, but it needs various strange Gtk settings to tell apps to use it, and I can't get most of those to work.

There are some impressive KDE-based distros out there, and I wish someone rose to the challenge of offering a Mac/Unity-like Xfce-based desktop.