I'm sure this has been talked about before, but I think the ideal of what a scroll bar should try to impress on the user is that it is a shading, representing the area they currently see, over the entire bar, representing the entire length of the page. Basically similar to what Sublime does, showing you a zoomed out view of the text with a shading over the section you currently see. http://i.stack.imgur.com/UeGnE.png
Obviously, this is pretty much how the size of the bar is calculated already, but few designs actually translate it very well. That's what really struck me about the iOS and OS X Lion scroll bars over the others he showed. They really do a much better job at actually conveying this to the user, or at least, they did for me. Ubuntu's terminal scrollbar is also similar in this regard. It would be awesome to see default designs start to focus on this in the future.
Obviously, this is pretty much how the size of the bar is calculated already, but few designs actually translate it very well. That's what really struck me about the iOS and OS X Lion scroll bars over the others he showed. They really do a much better job at actually conveying this to the user, or at least, they did for me. Ubuntu's terminal scrollbar is also similar in this regard. It would be awesome to see default designs start to focus on this in the future.