This is a very personal choice in my opinion. I think people who have a high keyboard usage rate like programmers tend to prefer Markdown as means for writing and formating their text. Traditional document editors like MS Word and even cloud alternatives like Google Docs rely largely on cluttered menu bars with the mouse hovering over a labrynth of buttons and drop-downs.

Dropbox paper brings the best of Markdown while keeping the extremely basic elements from classic word editors. It is minimal and doesn't bog you down with a host of complex page layouts and templates. The extreme minimalism is the major attraction point for it.

It also serves as a very basic presentation tool too and comes packaged with subtle transition animations. If I have the freedom to chose the platform for any of my presentations I would definitely choose this.