I just resolved this issue on my laptop, I'll share here and hope it works for anyone left watching the thread:
I was using Emerald themes before the upgrade, some people were saying their themes weren't working and they were told that Unity didn't fully support Emerald, or xml themes yet... so I thought stop using Emerald with unity for now.
1/ Open teminal
2/ Type: cd ~/.config/compiz
3/ Type: gedit compiz-manager
4/ Make sure it only says - USE_EMERALD="no"
5/ Save the file, and log off, and on again!
And I got my close restore and minimize buttons back, it looks nice again!
Hope this helps someone!