It's not just setting up a private email server, but the fact that she did her work as Secretary of the State through that private email address that is the problem. There is a reason why you are given a work email address when you are at work: so that it could be monitored and tracked and the company (govt in this case) knows that you are doing your job correctly.
If everyone used their private email addresses as their work, there wouldn't be any way to track down those employees who are specifically doing illegal things on behalf of the company's time, expense, etc.
This is a huge matter of trust, and I just don't understand how a candidate can run when he/she has this controversy behind him/her. Almost as bad as the Nixon tapes...almost...and it should be treated as such (the only it isn't is because Hillary as the backing of the DNC and they will do all they can to make sure she becomes the nominee, which means silencing this matter as much as possible....)