I've heard it said the only difference between a man-rated rocket and an unmanned one is the pile of paper work attached.

Some interesting history: The Soviet Union's Proton rocket was originally developed to launch a manned version of the Zond spacecraft to the Moon for lunar orbit and return. Unforunately the Soviet engineers could never get the Proton "manned rated". I don't know what the Proton's problem was. However the American Titan-2 was difficult to man-rate due to the pogo problem (it's something like water hammer in the fuel lines). There was a fair amount of tension between NASA and the USAF over this. The USAF was happy with the Titan-2 in its original form as an ICBM but NASA kept pushing them to spend more money to deal with the pogo issue. Eventually the problem was solved and Titan-2 performed well for Gemini. IMHO, they should still be using assembly line manufactured Titan-2s as low cost ELVs.

Gary