Was it really the icon's graphic design, or the fact that the word "free" is positioned very prominently within it? It'd be an interesting experiment to actually change the app icon for a month and test the results. Of course that's risking real money, so it'd be understandable if you didn't want to take the chance. :)
Good question- it's both, and the color scheme. If you pull up the Book category, most of the icons suck- plain, generic, bookcase after bookcase. So, a big red icon that also states FREE prominently is sure to get stand out.
Based on how we moved up the list, I attribute most of the sales growth to increased visibility from our icon as we moved up the Top 100 list, assuming that search traffic from things like 'Free Books' would remain relatively steady.
