The Clinton's are no strangers to scrutiny, and the latest revelation regarding Hillary's private e-mail server and the fact that she used it exclusively during her time as Secretary of State and had direct control over which communications to release as "relevant" to her time in that office seems rather indicative of her mindset. There is a certain egotism and narcissism that seems virtually required nowadays in anyone that wants to be President of the United States, and of course a need to control things. It seems pretty clear that based on the timing of the creation of the e-mail servers, Clinton fully intended to keep control of her e-mail communiques. The fact that she only got around to sharing her e-mails after she had left her position as Secretary of State and Congress had requested her records two years after her tenure shows that she:
a. Never really considered the law and the requirement that her communiques be handed over to the National Archive, Basically she didn't think it was important
b. She hoped that the records would never be questioned, or that they could at least be kept under her control indefinitely.
c. She felt that it was important to keep control of her messages, so she (or persons she trusted) could review them and seek to avoid any unflattering or politically damaging information from coming to light.
d. As always, there may be some combination of the above, or of course some other reason that hasn't been posited.
Now, Hillary has claimed that it was more convenient for her to handle her e-mail using this method, so she could do her copious personal e-mailing and Secretary of State e-mailing using the same account. That rationale seems pretty thin though. With the volume of e-mail that went through her account, it seems likely that she had to have some way to sort through them when reviewing them (or I guess when her aides were reviewing them). Her reassurances that the system was secure sound quite "Clintonian" since she isn't giving any specifics regarding what safeguards were in place, but she does assure us that the house where the server resided was protected by the Secret Service. We can rest assured that no one broke into her house and stole her server. As far as threats from remote hackers, as well as questions as to what sort of backup methods were used for the server data remain unanswered, and she currently appears to be resisting any investigation of those safeguards (or lack thereof).
Is this the sort of action that should damage her credibility, or viability as a Presidential Candidate? In the real world, not likely. Hillary supporters will continue to support here, and haters are still gonna hate. There is no particular challenger to Hillary at this point, at least for the Democratic nomination (the Republicans have a weak field at best). She isn't going to be prosecuted over this, but it is probably troubling to those that who aren't particularly Hillary supporters, but can be potentially won over.
It seems that the best spin on this is that Hillary was both arrogant and lazy, and either didn't have any advisers that were willing to warn her about how her policy could be viewed, or that she wasn't willing to accept that criticism/overrode it. The thing is, the type of person that manages to become a Presidential Candidate is also the kind of person that is likely to have that kind of arrogance, so a lot will have to do with who her challenger is. There are worse spins on this of course, but it doesn't seem possible to completely write this indiscretion off. The only question is how serious you see it as being.
I expect this will blow over, unless some further revelations come to light. If some e-mails wind up being released that were not turned over that at least imply that she was seeking to avoid detection of something more sinister, but at this point that seems rather unlikely. Still, if some enterprising hacker out there does have that sort of information available, they could be waiting to reveal it at what they consider a more opportune time.