There is no problem with her using a private email server. I've got one in my basement, and I've had it for more than 8 years.
The problem is that she was using her private email server for federal business: first, to evade public scrutiny, which is illegal; second, for sending and receiving classified documents, which is also illegal.
The Colin Powell defense is, as I already said, bullshit. It is true that he had a non-government email address that he used to conduct government business when he was SOS, but there are three differences. First, he didn't use it for classified work - he carried two devices, as we say now. Second, the State Department didn't have a non-classified email server at the time. Third, he used a commercial email service, which means that in the event of a discovery request or subpoena a third party would be in charge of providing the records, not his own lawyers and cleaning crew.
Go read about his email leaks. He tried repeatedly to warn Hillary against using him as her defense. She eventually got the message but, like the Japanese submarine from Gilligan's Island, some of her sycophants are still fighting that lost war.
Public records from the Bush administration were a fucking mess because so many of the departments were on their own for providing email services. In response to this, a bunch of new domains and email servers were set up for official use, and the use of outside email for public business was banned. Once that was complete (2009?), Hillary even sent emails to her staff telling them not to use outside email services for state business.
P.S. Obama should indeed have noticed. StoneTear's reddit request was probably not about obscuring Hillary's email address, but Obama's.
P.P.S. Oh, and her staff knew that their boss was breaking the law, but were told to keep their mouths shut. If we'd learned about this from a whistleblower years ago, instead of from a lawsuit last year, maybe this would have blown over by now and not been a huge topic in her election campaign.