I don't know much about the specifics of the mechanics of governments, but what I've gathered from the posts in this thread is that progressive changes with the actual amount of income and flat doesn't. I personally don't really see what's wrong with flat, assuming it is what I think it is. If it's the same percentage for everyone, then it's automatically gonna scale with the amount of money they make. So why increase it even more for rich people? Don't get me wrong, I don't like rich people any more than the next guy, but still.