Select the folder you want to expand in Explorer and then press the numeric keypad plus (+) key to expand a folder's subfolders and then use the numeric keypad minus (-) key to collapse them again. Works in folder pane only. For mouse-centric user, remember there are keyboard oriented people who naviate with keyboard only.

I started very keyboard oriented (as a touch typist), but have moved towards mouse usage more and more. You can do everything with the keyboard but it gets very ugly. On such issues, I tend to be pragmatic, not religious.