One of the nice things about Linux desktops is that you're not tied to the default window manager like you are in Windows and OSX. While Linux distros might have a default window manager (like your Unity example for Ubuntu) it is still easy to switch.

This allows developers to experiment with different paradigms with regards to the GUI, see for example the popularity of tiling WMs on Linux. (i3 has my preference, others swear by xmonad).

I know it is possible to change window managers for Windows and OSX as well but the experience is just not the same.