Just today I was wishing that Chrome's Hold-to-Quit functionality was an OS-wide feature. Thank you very much for creating this, it was an instant purchase in my book.
One thing that Chrome doesn't do is even when there are zero windows open, you still have to hold down Command-Q to quit. If no windows are open, there doesn't appear to be any danger in quitting right away--after all, the major mis-press is when you try to hit Command-W to close a window and instead hit Command-Q, but if there are no windows open, there's no reason to be hitting Command-W.
Screenshot: http://paulirish.com/i/fcba70.png