As for “horizontal progress for vertical content is bad”, I’m pretty sure that’s right, but that’s mainly relying on my intuition. If I think about the concept and scroll my page up and down while imagining a horizontal scroll bar, my mind just rebels and thinks “this is not right”. It’s just obvious to me that a scroll bar should be vertical when you scroll vertically.
I can also offer some reasoning: when the scroll bar is vertical, it moves in the same direction as the direction of your scrolling movement on the mouse. This makes it easier to mentally connect your fingers’ scrolling movement to the scrolling on the screen, which makes the device use easier and more intuitive. In the case of iOS and OS X 10.7 and later, your finger movement is in the same direction as the content rather than the scroll bar, but it’s still something on the screen moving with your fingers. But a horizontal scroll bar moving when you push your fingers vertically is too abstract a concept for your mind to intuitively grasp, so it’s harder to use.
I get 100% why it seem like it should matter I just don't buy that it actually does matter (much). http://www.businessweek.com/articles/2012-11-29/the-science-.... That article is a great example of how completely wrong our intuition can be when it comes to optimizing conversions and I think the same applies to usability.
I can also offer some reasoning: when the scroll bar is vertical, it moves in the same direction as the direction of your scrolling movement on the mouse. This makes it easier to mentally connect your fingers’ scrolling movement to the scrolling on the screen, which makes the device use easier and more intuitive. In the case of iOS and OS X 10.7 and later, your finger movement is in the same direction as the content rather than the scroll bar, but it’s still something on the screen moving with your fingers. But a horizontal scroll bar moving when you push your fingers vertically is too abstract a concept for your mind to intuitively grasp, so it’s harder to use.