My only guess could be if the front panel connectors are hooked up wrong and all of the other cables are plugged into the right spots. You could try double checking those if you haven't already. Otherwise, see if you can get a cheap PSU from a local tech store to test then return it if the problem persists. 

Unplug each cable from the PSU itself, and verify both that it's plugged into the correct slot, and that the correct end of the cable is plugged into the PSU. Having the cable reversed can actually cause a direct short, and is one reason why the unit may be refusing to power on. Some people do accidentally reverse the EPS12V or the 6/8pin GPU cable at the PSU, even some PSUs I own allow them to be plugged in backwards. Then double check all the cables are correctly lined up with whatever they are plugged into one your system. 
 
After all of that if the problem still occurs, take everything out of the case and try to power it on without the case attached to anything. Be sure to use the motherboard box under the motherboard, or something else nonconductive. Anti-ESD bags are actually bad for this, don't put the board on the bag it came in.

I/O was not plugged in properly. We have power! Just need to wait for a GRaphics card now as mines apparently too old for the MB.