I really have to reuse this post because I have the same issue and perhaps found another way to solve it. But I want to understand why this way may work.
1.) Stuck in bootscreen - loading indicator spinning forever.
2.) Tried every hint of this post. Nothing worked.
3.) Tried to boot the Mac with a bootable backup (via SuperDuper) of another Mac on an external HDD - worked just fine.
4.) Did a backup of the "broken" Mac on external HDD via SuperDuper.
5.) Tried to boot it on another Mac. Worked just fine!!!
6.) Tried to boot it on the "broken" Mac. Worked just fine!!!
So why does the exact copy of the system boot from external HDD and not from the internal one? I think there might still be issues with the permissions. When I tried to fix them on the "broken" Mac, there were several error messages, telling me some permissions were not able to be fixed.
My idea now: Format the HDD of the "broken" Mac. Clone the backup of it that I put on the external HDD back to the HDD in the "broken" Mac via SuperDuper. Perhaps it repeats its magic and everythin will work fine again.
Or you know by reading my descriptions, what this magic is and I can perform it via terminal or disk tool without having to erase my system?
I'm looking forward to your reply and thank you a lot in advance for your help.
P.S.: MacBook Pro Mid 2009, 2,66 Core 2 Duo, 4 GB RAM, 320 GB HDD, OS X 10.9.2