It seems that I have to keep telling people. The sites not working correctly in IE11 are problems with those sites... not problems with IE11.
Many sites do fancy things depending on what browser you use, and they don't know about IE11 because it uses a totally different user agent string, so those sites fail to render correctly.
This is easy to prove. All you have to do is hit F12, and go to the Emulation tab, then set the User Agent String to IE10. All this does is make IE tell the site that it's really IE10, and voila.. the site magically works.
Those sites need to update their code to be compatible with IE11, not the other way around... although Microsoft does have a means to provide compatibility lists for sites that refuse to update themselves.