This could be great news for the democratic party, if a better candidate replaces her. Shouldn't be hard, and Obama kinda came out of left field last time.
I suspect Hilary is a victim of a typical older person's vice: treating email like a secure and private conversation. Always assume email is public, especially when it is business email. Even with private email, the NSA sees far more than you would like to believe. The internet is not anonymous if someone's motivated enough.
I agree with Rhizo that this will probably end up swept under the rug. Petreus was still a semi-outsider in Washington and posed a threat to the political ambitions of others. Hilary's far too ensconced in the existing power structure for her to see any real consequences. In any major executive culture, there's plenty of blackmail to go around if one person is threatened with prosecution. At the most, she'll have to trade her candidacy for leniency. Depends on whether the Party apparatus has a backup candidate.
JacobZuma, Republicans, and especially the yellow-dog/tea-party variety like Lysistra, have always hated Hilary since her plan to implement Obamacare in the 90s. She represents every liberal value that conservatives, especially men, hate. At the start of her husband's presidency, she was muzzled for being too influential in the administration. It's my belief that she has always been the main driver and power behind the Clinton presidency.
Of course, just because someone is hated doesn't mean it wasn't earned. With a career as long as hers, her enemies stretch from sea to shining sea. It's part of the reason she can't "pull an Obama" and use her minority status to block criticism. Critics of Obama have to be careful that any commentary on him can't be spun as racist. Hilary, however, has enough history that safe criticism of her is instinctual at this point.