Design is hard because there are many small decisions that lead to a great-looking page. Everyone knows the concepts: white space, big images, good-looking fonts, but even with that knowledge it is hard to get it right. Pretty much like composing music: everyone can master music theory or playing an instrument with a little effort, very few can compose master pieces.
Very often, I email a deck back to a client at the end of a project and say "I might a few very small changes, you probably won't notice them". But, these small changes added up can make a big difference to a layout.
So what is it I do in the final stages of a presentation design project? It is hard to capture.
The main thing I think is completely stepping away from the actual content, i.e. the text that is written in boxes. Instead I see black/light grey patterns of characters, boxes in different shapes and colors. Almost "squinting" at the page and making adjustments until this "cubist painting" looks right in terms of proportions and balance.
Most of the times, it work but yes, there are pages where even me as a professional simply cannot get it right.
Image via WikiPedia