Following the WP 3.5 upgrade to one of my sites, I have had to install CKEDITOR to get proper editing of posts. TinyMCE and Ultimate TinyMCE (which I was using) just don't work any more. Inserting links does nothing, can't switch to HTML view, lots of other issues. Removed the "ultimate" plugin and deleted it, but the standard editor has the same issues. Switched to CKEDITOR (which I hate with a passion) and everything works again.
I believe this issue may somehow be related to the use of custom fields and/or custom post types. The editor works ok on my other upgraded sites, just not on the one using custom fields and custom post types in the theme (GeoTheme). There was a similar issue earlier this year with ACF (which I don't use).
Gonna have to downgrade at this stage, which is a shame.