I uploaded a patch to test more elements.
In addition to the comment form input fields:
- the
gallery shortcode output would lose the column grid
- the
caption shortcode would change its width (possibly only a minor difference)
The classic navigation widgets looked the same with the nav element at first glance: Archives, Categories, Meta, Navigation Menu, Pages, Recent Comments, Recent Posts, RSS and Tag Cloud. For the search form, Twenty Eleven has its own searchform.php template.
Child themes that replace the main stylesheet would need to update their styles if the theme support is added (unless the theme also overrides the twentyeleven_setup() function or the site does not enable comments).
I think it would be worth at least adding the two input types in the stylesheet for the comment form, even if the support is not officially added in twentyeleven_setup().