A slap on the wrist? It ended his career. (Any charges, etc, were a means to that end, but he resigned.)

His public service career may have ended, but he got gigs at several private firms, as well as Harvard, Exeter, and several other universities.

I think most people believe it's unlikely Clinton will serve any time for the email lapse; instead, she's considered the front-runner for the White House.

On the other hand, Snowden had to flee the country and, like Assange, he may never be able to set foot in a country with the relevant extradition treaties.

I think the high/low court point still stands.

Snowdens's purpose was to undermine the intelligence apparatus of the US, while that clearly was not Petraeus's intent.

Undermine?

You mean: Disclose that the intelligence apparatus is spying on its own citizens/taxpayers who pay their salaries?

There, fixed it for you.

Compared to possible life in prison, possible capital punishment, possible gitmo, currently exiled in Russia, I'd say losing one's job is a comparative slap on the wrist

True, but the magnitude and scope of the disclosures were far different. One would reasonable expect the (potential) punishment to be different.