"Ryan: Why use a slideshow at all? Select multiple files and hit spacebar, then just use the right/left arrows. Or just hit one, then up and down. But indeed, pretty much any automated slideshow is troublesome."

Because I like the full screen functionality, and I suppose that to someone at Apple, full screen == slideshow, which they also equate with auto-advance. Whenever you hit the "full screen" button, the OS helpfully pushes the "play" button for you. This is one example where Apple got it wrong *because* they sweated the details.