Hillary is obviously stepping down to prepare her presidential bid, and to make sure she does not getting Obama stain on herself in the next 4 years. She had to take the fall for Benghazi, and I doubt she did that willingly - trade off? Take 4 years off, then become The Queen of America...?
Holder - About to be exposed for multiple crimes, will instead "retire" and likely wait for everything to cool off so that he may serve in the Queen's Court.
As noted in other replies, on one gives a crap about affairs any more, certainly not Hillary. In 4 years, Petraeus will be a hero for having taken this step, and he, too, will be given a prime post in Hilary cabinet. He is not being punished for his affair, he is being rewarded for his other crimes and this is akin to a "plea bargain" with the public.
This is the article that got my posting privileges revoked: