Keyboards
Turns out it also borks the touch keyboard. If you have it set to something that's actually usable (ie, the full size one with useful keys on), kiss goodbye to it after you've logged in. You get lumbered with the useless version designed more for phones (and this is with my tablet set to Desktop mode).
Happily, there is a fix. Rename the 'ink' folder in Commin Files, and copy over the same folder from your previous install. Restart, and you have a useful keyboard again. Apparently, and according to an MVP post, the full keyboard has "issues" with the new update. Yet it works fine to log you on, and works fine after you've put it back.