These two-dimensional presentations confuse me if I don't realize they are two-dimensional at first. I press the right arrow key through the entire presentation and I find it much shorter than I imagined. Then I realize you have to go down as well.
Don't think too much, just hit 'Space' and it will guide you correctly through the slides.
Altough even the guided tour feels spatially incorrect...
Good intentions, awful execution.
On the other hand, if the slides are intended for reading, then yes, they may be a usability disaster. But then, slides aren't going to be the best format for reading anyway.