Didn't like taking away some items that I counted on in Twenty Eleven like the "Posted on" function and taking away the 3 widget areas in the footer and the search box in the Navigation bar.
You should have made them optional for us to have or not - not just remove the code without a heads up.
Also not a fan of how hard it is to override the styles and colors of some of the CSS selectors like widget links and caption text. Even using !Important doesn't always work.
Love the ability to use external image links in the media button to save on coding them myself but since I can't easily correct the code that shows up it is a wash. The alignright alignleft classes don't work in the RSS feeds so images look wrong.
The new REM sizing is hard to learn and seems to add to the size of css files but I'm glad to get the chance to learn this new standard before I have to use it.
I like the mobile first focus but really could use some tutorials and guide on how to create and edit my child theme to take advantage of the new gizmos in 2012.