Are you using IE9 and running it in compatilibility mode? If so that makes it render as if it’s IE7 and IE7 is not supported.
Are you using Do Not Track Plus or Ghostery or any such private browsing add-ons or extensions?
Have you tried disabling all browser add-ons and extensions?
How are you connecting to the internet and to WordPress.com?
Exactly which browser and version of it are you using?