Twenty Thirteen is not a simple theme to work with if you are planning to use it as a base for customization with child themes.
Main issues to me, so far:
- Header image
- Main navigation menu
- Footer widget area
Header image is set to a fixed size, it gets chopped off for mobile devices. To get around this you'll need to re-write the custom header function and its styles.
Default theme style for main nav uses display:block-inline for ul li elements, creating gaps between them. You'll have to optimize the html render, deleting whitespaces and breaks to eliminate the gaps. Check the Output Otimizer plugin, it does the trick.
Masonry sets a fixed width for footer widgets. If you ever want to use more than 4 columns or justify any number of columns for the footer, editing the theme styles won't do it. Another instance where you'll have to cut through scarce documentation and obscurity to edit simple CSS.
support thread on the masonry js script for footer widgets
Twenty Thirteen's wireframe is a very nice, well thought and clear layout of the html boxes, but looks can be deceiving. A no-brainer theme to be used as default, but a heck of a challenge for customization.