Comey's explanation was odd and unpersuasive on its face. He began by reciting what the law requires for a felony or misdemeanor conviction in cases like this. He noted that gross negligence is the standard for a felony conviction. He then recited the facts as the FBI found them. These facts suggest gross negligence.
When it came time to merge these two strands and present his decision whether to prosecute, Comey made no reference to the legal standard he had articulated a few minutes earlier. Instead, he pulled a switcheroo, formulating a new legal standard based on the elements he says have been present in past cases where prosecutions have been brought for the mishandling of secret/classified information. Gross negligence exited stage left -- replaced by intent to harm the U.S., disloyalty, etc.
I don't know whether Comey is correct about past prosecutions. For example, did David Petraeus' case meet the criteria Comey articulated this morning. Maybe, since he intended that his mistress see the information he showed her. But more than Comey's say-so should be required before we buy his assertions regarding past (and present prosecutorial) practice....
In any event, Comey simply ignored the statutory standard he laid out. A first year associate at a law firm wouldn't dare present an analysis like this.
I would add only that the no-indictment recommendation will cement the dark and corrosive narratives Trump has been peddling in a way that Trump himself has been unable to do, try though he has at his screeching best.
What's Trump been saying? That Hillary is dishonest from morning till night, that she gets by with behavior that would land lesser folks in the slammer, that she builds her empire (and her wealth) with connections ordinary people have no chance of getting; and that the system is rigged (say, by having your husband get a closed, untranscribed meeting with Comey's boss four days before the Director's announcement).
The whole problem with Hillary is not so much that she's a crook, which many people (not just Republicans) have thought for years. The problem is more galling and, now, less escapable: Hillary is a crook who gets away with it.
Whether she gets the Presidency, however, remains to be seen.