Basically, Expat-like licenses do not use the copyleft system: When you distribute a project that’s copyleft licensed, you must give the exact same rights you were given (including the source code, license terms, etc). To resume this in 1 quote “the rights of one ends where the rights of the other begin”
From what I’ve heard, openSUSE Tumbleweed is the most stable rolling release distribution around. It automatically checks packages, before releasing them. As for desktop environments, Xfce is a great one, if you add some addons (e.g., Whisker menu).