As many of you may know, Microsoft just rolled back SP1 for SharePoint 2013! Besides, SP1 is not a trivial update and will likely require an entire rebuild of your SharePoint farm.
The much easier fix is to install the following hotfix: KB 2600088
The hotfix updates these browser definition files to correctly detect IE 11's user agent string, allowing SharePoint to function normally with this browser.
The best way to test this theory is to hit F12 and open your IE developer toolbar when viewing your SharePoint site. If you change your User Agent string to IE 10, the site should function normally. This highlights the issue that the hotfix remedies.