Wow, I had never noticed that shadow thumb until this article pointed it out. I had to go back play with it. A couple other random observations: when you click on one of the arrows the real thumb doesn't snap up to the shadow thumb until you've moved the cursor a certain distance away. Also, scrolling by clicking and dragging the thumb has the lag effect mentioned in the article, but scrolling with two fingers on my trackpad doesn't.
I think the other problem that isn't mentioned in the article is that, at least for long waves, loading of content seems to make scrolling jerky and unpredictable.