2009 iMac won't boot after clean install

I ran the initial release of Yosemite just fine (updating from Mavericks) until just before the first Yosemite update release until one day my machine starting hanging at bootup with a spinning beachball at about 25% of progress bar. I scoured the internet for fixes and tried them all to no avail. I then simply decided to try a clean install of Yosemite.

After clean install, my machine still hangs at bootup and the only difference is that I get no spinning beachball. There is simply a mouse cursor on a blank grey screen after the progress bar reaches about 25%.

I started to compile a list of everything I did to solve the problem from the beginning but it seems irrelevent given that my machine is now on a clean install. I apologize in advance if my post is too vague.

So... Where do I go from here?

Message was edited by: Bad Sir Ivan I feel that I should have mentioned that I have run the hardware diagnostic from the original application install dvd that came with my system. I have also verified and repaired permissions and ran disk repair. No issues found.