Just throwing this out there as perhaps a suggested article or post idea for someone.
It would be great if a clever designer / typography type person put together some suggested combinations of the Custom Fonts available in WordPress.com.
There are many similar articles on Google Font Combinations, but nothing I could find on WordPress.com.
Seems like a good time to do it too with the new Custom Fonts being available. (http://en.blog.wordpress.com/2013/11/07/new-custom-fonts/)