I'm using WordPress 3.4.1 on a blog and had the problem that my visual editor was not working - no HTML/Visual Editor Tabs showed up.
Because I could not find a solution (it had nothing to do with profile-setting, plugins, cache, theme, tinymce, etc.) and I saw that some users in the forums had the same problem and the solutions for plugin/cache/theme related problems did not help I thought I might type up what caused my problem and how I fixed it.
The data in my installation is very old - the first admin users where added 2007.
I found the problem when a new user (two weeks old) told me he could switch between visual and html editor without a problem.
Well, maybe it might have had something to do with the visual editor setting in the user profile, but switching the option there on and off didn't change anything.
My solution was to look into the MySQL tables. In the 'user_meta' table i saw that the new user had a meta_key called 'rich_editing' with the value of 'true'. The old admin users did not have this key. So i added a new key rich_editing = true for the old users and i instantly saw the tabs to switch between visual and html editor.
Hope this helps some. If you don't know how to edit stuff in your mysql database or if this is not the solution to your problem I can't really help you, sorry. I just wanted to share this solution for this particular cause of problem.