I've updated to 3.5, as have my clients.
The list so far of issues (with all plugins disabled) is:
widgets don't work (dropdowns/widgets don't work as they should re:drag and drop)
can't update posts - issues with blank screens and editors
menus not expanding in the admin.
Everything in the front end appears to work for now, though one client is also reporting that her ajax commenting system isn't working. Doesn't matter which theme is running, and twentytwelve isn't working on either server. Switching to 2011, updating fresh (manually) with everything switched off and running 2011 makes no difference.
Adding the debug line makes no difference.
As one of the blogs is a bare wordpress install, I'm wondering what's wrong with the update perhaps?
Anyone else seeing this behaviour?