Return of the Mobile Stylesheet
At least 10% of your visitors access your site over a mobile device. They deserve a good experience (and if you provide one, they'll keep coming back).
Converting your multi-column layout to a single, linear flow is a good start. But mobile devices are not created equal, and their disparate handling of CSS is like 1998 all over again. Please your users and tame their devices with handheld style sheets, CSS media queries, and (where necessary) JavaScript or server-side techniques.