Also, with the Dock hidden, you don't know where your click target is until the mouse arrives at the relevant screen edge and you've waited for the icons to sli

Also, with the Dock hidden, you don't know where your click target is until the mouse arrives at the relevant screen edge and you've waited for the icons to slide in, at which point you still have to locate and click the icon you want. It sounds like a small thing, but it annoys me enough that I never hide the Dock.