Hillary used a private email server to send and receive emails to people who were approved to send and receive such emails, a few of which are being researched whether to grade them classified or not. Her predecessors did the same thing. Scott Walker has a private email server too. This is a witch hunt by the opposition and they've sunk to new depths with the Petraeus comparison.
Haven't really followed the case, but from what I read in German newspapers, (they seem to think that) the actual issue seems to be how she handled the case/accusations, not necessarly whether she actually used the private account for classified information?
That said, I find it weird to handle business via a private e-mail. Seems like an absolute no-go (no matter the classification) to me. I think there's even some notes about that in our code of conduct. No idea how common that is for government officials (in the US or my country) though. That said, whatever she did is obviously not comparable in any way to what Petraeus did.