This ties into what I was talking about above with respect to "heavy algorithmic stuff". Most developers can work with intricate legacy code by adding an if here and there to handle special cases. Very few can recognize that it's encoding a state machine with state transition function Y, and rewrite it. Even then, very few have enough 'attention to detail' to do the rewrite, and NOT lose any legacy functionality. This is an example of what I consider to be 'hard stuff'.