Username or EmailPassword
but as an example, let me mention the Windows right-click drag'n drop. It's something that won't limit any new user to do anything. it's something that it's there, to be used as long you know it... and know what? It's a LOT intuitive after you tried it (like learning to ride a bicicle!) and a real timesaver... non obstrutive pop-ups... it's safe (if the user don't know exactly what the action will do, the right click drag'n drop show him/her the options before doing anything... or nothing at all!).
Actually, Windows' right-drag does get in the way. Because of the right-drag thing, Windows can't immediately display a right-click context menu on button press. It has to wait for the button release.
Also, you get the same functionality with middle-drag or Alt+drag in Nautilus.