The primary issue that was revealed, though not through Wikileaks, was her unlawful use of a private server to conduct State Department business.
Hillary's leaked emails reveal major multinationals contributing substantial sums to "Bill Clinton Inc." and the "Clinton Global Initiative", sometimes for face-to-face meetings with the former president or speeches. We also found out her own team was critical of her "terrible instincts", Mr Podesta complained that Clinton's personal lawyer David Kendall, as well as former State Department staffers Cheryl Mills and Philippe Reines concealed those detrimental 'instincts'. It was also revealed that despite Obama claiming he found out about her private server "from the news", he had email exchanges with her not addressed "state.gov". Her leaked emails show her participation in a "Clinton Global Initiative" summit held in Morocco was contingent on a donation of $12 million from the king there. We found out she described Bernie as a "doofus". It was revealed that Podesta strategized on Hillary's vice-presidential selection with a list of potential running mates arranged as what he called "rough food groups" referring to demographic coalitions. Clinton campaign manager Robby Mook emailed Podesta about rescheduling the Illinois primary and noted "the Clintons won't forget what their friends have done for them". According to the leaked emails, Hillary told a Goldman Sachs conference she would like to intervene secretly in Syria, and that she would deploy missiles around North Korea if China didn't disarm them. Her communications director and a think tank fellow discussed the "bastardization of Catholicism" referring to it's prevalence among conservatives. She referred to her need to have a public and private position on whatever issues. We found out CNN contributor Donna Brazile provided Hillary with questions she would be asked in advance of a Town Hall event the broadcaster was hosting. Brazile took over at the DNC when Debbie Wasserman's emails revealed the DNC sidelined Bernie. Her campaign spokesman, Brian Fallon noted the DoJ meeting to discuss action on her unlawful private server use and transmission of classified materials suggesting some sort of collusion. Hillary's emails indicate her concern over admitting Muslim refugees. She expressed a desire for a "hemispheric common market, with open trade and open borders". Robby Mook expressed his concern over the fact one of the Keystone Pipeline project's investors was also a substantial contributor to her campaign, and the need to deal with her environmentalist constituents. One of Bill Clinton's advisers described daughter Chelsea as a "spoiled brat". Podesta refers to "needy latinos and one easy call" suggesting Hillary reach out to former New Mexico Governor Bill Richardson and former Energy Secretary Federico Pena to ask for their support in her primary campaign.
None of all this is particularly awful, one would be reasonably critical of it all, but it isn't earth-shattering, however, what if only Putin knew about it?
Hillary's emails are here:
https://wikileaks.org/clinton-emails/