From the FBI's press release:
Although we did not find clear evidence that Secretary Clinton or her colleagues intended to violate laws governing the handling of classified information, there is evidence that they were extremely careless in their handling of very sensitive, highly classified information.
...seven e-mail chains concern matters that were classified at the Top Secret/Special Access Program level when they were sent and received...
There is evidence to support a conclusion that any reasonable person in Secretary Clinton’s position...should have known that an unclassified system was no place for that conversation.
we also found information that was properly classified as Secret by the U.S. Intelligence Community at the time it was discussed 
if information is not marked “classified” in an e-mail, participants who know or should know that the subject matter is classified are still obligated to protect it
Clinton was reckless, negligent, and/or incompetent and disqualified.