Auto-hiding scroll bars. How can you tell whether or not you should scroll up or down to reveal more content in a list? Sometimes it's easy, other times it's not. You certainly have no way of knowing where you are until you start to scroll. I've disabled the default in favor of always showing scroll bars (the old behavior).
This changes based on input device too. A system with just a touchpad will by default hide them, just as an iPad/iPhone/Android device does. A system with a mouse will show them by default. And on a laptop with a touchpad, they will unhide if a mouse is attached, and hide again if the mouse is unplugged.