I think part of making this solution intuitive is to make the submenu box a a fixed height that's equal to the first menu. It's what the author changed his menu to look like as well. Amazon got around having too much empty a space by adding pictures, and too many submenu options by adding a second column. If your submenu is so long that even 2 or 3 columns isnt' enough, it would have been too long for a single column anyway.