She used a private email to server to hide her shady dealings, some classified information was discussed as well. It seems to sound worse than it is, but the handling of classified information is treated very seriously by the government so some think she should have been penalized just as a 19 year old soldier would for the doing the same.
Clinton lied about it while under oath and destroyed evidence under federal subpoena, which bothers some. She then explained it away through ignorance of email, which bothers some.
Other emails suggest the DNC rigged the primary in favor of Clinton, and that the Clinton's use their foundation to launder political bribes.
Another angle is that the FBI is leaking this information out because they want to hurt her campaign. At the very least, this suggest Hillary's "strong leadership" while Secretary of State is in question.
The reality, though, is that if this wasn't during campaign season no one would give a single fukk. Not sure if that speaks more to Hillary's actions or the American public, though.