New devices are coming out almost daily. Make sure your site performs well on all of them.
The majority of website traffic comes from handheld mobile devices. Make sure your site caters to this majority demographic.
A site redesign is an ideal time to revamp your content.
As long as you are updating the look and the content, you should make sure that it is easy for your users to access that beautiful content. Re-evaluate your sitemap and call-outs.
Even your content is up to date, if the style of your site has been passed up by current trends your content will appear out of date.