The only possible way to answer this is to quote more from the official text of Comey's press release. I've added my own emphasis, however.
Although there is evidence of potential violations of the statutes
regarding the handling of classified information, our judgment is that
no reasonable prosecutor would bring such a case. Prosecutors
necessarily weigh a number of factors before bringing charges. There
are obvious considerations, like the strength of the evidence,
especially regarding intent. Responsible decisions also consider the
context of a person’s actions, and how similar situations have been
handled in the past.
In looking back at our investigations into mishandling or removal of
classified information, we cannot find a case that would support
bringing criminal charges on these facts. All the cases prosecuted
involved some combination of: clearly intentional and willful
mishandling of classified information; or vast quantities of materials
exposed in such a way as to support an inference of intentional
misconduct; or indications of disloyalty to the United States; or
efforts to obstruct justice. We do not see those things here.
To be clear, this is not to suggest that in similar circumstances, a
person who engaged in this activity would face no consequences. To the
contrary, those individuals are often subject to security or
administrative sanctions. But that is not what we are deciding now.
As a result, although the Department of Justice makes final decisions
on matters like this, we are expressing to Justice our view that no
charges are appropriate in this case.
Taking it point-by-point
- Comey (and the FBI) looked at other cases which were similar.
- They did not see any "clearly intentional and willful mishandling of classified information".
- They did not see "vast quantities of materials exposed in such a way as to support an inference of intentional misconduct".
- They did not see any "indications of disloyalty".
- They did not see any "efforts to obstruct justice".
Presumably, these are all things that previous successful cases have turned on.
It's a matter of opinion as to whether or not you agree with him on these points, but those are his stated, official reasons. Likewise, he was not asked to recommend sanctions (presumably because she is no longer a government employee) - he was only investigating whether there was enough evidence to support filing charges.
As my editorial (rather than personal) opinion, I think the last two are fairly clearly in the "not applicable here" category. The second one is probably in that category, since the material was not generally exposed (such as happened with Snowden). All the (healthy!) controversy should be on whether the "clearly intentional and willful mishandling" should apply or not, as your question asks.
Comey feels that it doesn't, and he has more information on the matter than anyone else; but that doesn't mean that someone else couldn't come to a different conclusion when presented with the same evidence. At this time, no one else has it all, however.
Update 7/7, after a Congressional hearing
As of yet, I haven't seen an official transcript to cite from, but this live blog from the New York Times appears to have two relevant direct quotes (emphasis mine):
Representative William Lacy Clay, a Democrat from Missouri, asked Mr.
Comey whether the former secretary of state had “intentionally
violated the law” with her email use.
“We did not develop clear evidence of that,” Mr. Comey said.
Did she obstruct justice, Mr. Clay asked?
“We did not develop evidence of that,” the F.B.I. director said.
And would her actions rise to a level that would warrant prosecution?
“No Justice Department, whether under Democrats or Republicans, would
prosecute that case,” he said. (ref)
Mr. Hurd said it was outrageous that there was not a recommendation to
prosecute.
“What does it take for someone to misuse classified information and
get in trouble for it?” he asked.
“It takes mishandling it and criminal intent,” Mr. Comey said.
“And so an unauthorized server in the basement is not mishandling?”
Mr. Hurd said.
“Well, no, there is evidence of mishandling here,” Mr. Comey said.
“This whole investigation at the end focused on: Is there sufficient
evidence of intent?”
Mr. Hurd, like other Republicans, raised the question of precedent,
asking whether the decision not to punish Mrs. Clinton sent a message
to others in the government who handle classified information. He
asked whether an employee of the F.B.I. should have a private server.
“They better not – that’s one of the reasons I’m talking about this,”
Mr. Comey said. He added, “There will be discipline from termination
to reprimand and everything in between for people who mishandle
classified information.” (ref)
Presuming this is an accurate transcription of what Comey said, it's clear that he believes that there was no intent (or at least not enough evidence to show it), and that demonstrating that there was intent would be required to successfully prosecute. Likewise, Clinton herself is not subject to any form of discipline other than prosecution, as she is no longer a State department employee and (presumably) no longer has an active security clearance to be revoked.
I would extrapolate from this that Clinton will have a hard time (if not an impossible one) getting a security clearance again, but that's not a concern for the President.
Update 7/29
I'm a little behind on my news-reading, but Ars Technica has a detailed, technical, and unbiased writeup of what happened, based on all the available public evidence (Comey, the hearing, FOIA requests, the OIG report, etc). I highly recommend reading it for the full context of why Clinton wouldn't have realized she was doing anything wrong, just how wrong she was, and what has been done to prevent this from occurring again, without straying into the debate about whether or not she should have been prosecuted. As for the repercussions, it also has the best summary I've seen:
As far as Clinton is concerned, if she is elected president, she certainly won't be using her own technology day-to-day. And if she isn't elected, she'll likely never have access to confidential information again—nor, potentially, will her former staff, who now face administrative reviews by the State Department.
