I'm less concerned with how this plays into her potential presidential run and more concerned about the potential for deception that this practice allows. It now seems Hagel also used a personal email address, and that the practice goes back to the Bush administration, and possibly further. There are clear rules for ALL fed employees, and that includes those at the top, regarding only using gov email and gov servers for all their correspondence.
The difference with the Clinton revelation (and it may end up applying to others) is that instead of using a personal email in addition to her gov email (which has come up before, with other officials), she used it exclusively for her peer to peer communications.