The feeling of "well, I'm scrolling, but nothing's happening. Am I doing something wrong...oh there it is" can really throw off an otherwise positive content experience.
Yeah... sometimes I'm scrolling and there's some sort of effect on the page that some design nerd spent a couple work-weeks on, but it may not always be readily apparent. If I do what I'm apt to do when scrolling doesn't work, aka click on the scrollbar to jump ahead, then it breaks the entire presentation. Having to scroll forever to get at the content I'm looking for is incredibly annoying.
The feeling of "well, I'm scrolling, but nothing's happening. Am I doing something wrong...oh there it is" can really throw off an otherwise positive content experience.