In short, everything that Clinton was accused of was completely accurate. Powerline had it right: "Alternative headline: "FBI declares Hillary Clinton to be complete liar."
I think this guy on Reddit, who describes himself as a lawyer, makes a good point:
If I were to play mind reader here, I would guess that the FBI's thinking is that if you're going to recommend charges against a major party candidate for president, you'd better be damned sure the grand jury will vote to indict, and that a petit jury will vote to convict. Otherwise it's a massive black eye for the FBI - perhaps the biggest in the history of the agency: they've changed the course of the presidential election only to fail to get a conviction. Comey was focused on the intent requirement during his press conference, so it appears they just didn't think intent would be a slam dunk before the grand jury and, if they vote to indict, the petit jury.So Comey didn't want to inject the FBI into the presidential race so he did the next best thing by saying that Hillary was "extremely careless" which - somehow - is not the same as "gross negligence." You have to be "super-duper careless" to rise to that level.