Given that it all appears to have been SECRET and not TS or TS with a codeword/SAP, it's only potentially "serious harm to national security", not "grave", and almost all of it was highly time sensitive. Flight schedules for helicopter missions I fly on are actually SECRET until they fly; afterward, they are basically public record. By waiting 6-12 months at least to release this information, the original leaker seems to have done a reasonable job of balancing OPSEC considerations.
The most serious allegations of leaked information so far are the diplomatic cables off JWICS, which would be potentially TS, and could cause lasting harm. As well, individual source names could be sensitive, although in my experience there is enough turnover among Afghans, as well as bad intelligence on the US part, that after a year or so, the names of any sources would be outdated anyway.
The newspapers can make a pretty reasonable first approximation of what in a body of information is particularly sensitive. This is all routine operational material, so the sensitive details are operational capabilities, individual sources, and other procedural details, items which actually have the least value to the public in being disclosed. The information which is most compellingly in the public interest is "what happened", which is already well known to any interested party. Given that this information was all relatively low levels of classification, there isn't anything super secret squirrel revealed here; anything super-secret squirrel would have been hidden from this classification as well.
I would like to see prompt declassification of as much as possible, and virtually every item in this release is something which should be declassified 1-2 years after the operation. Classification should never be used to cover up incompetence or mistakes or wrongdoing, only to protect national security.
There absolutely should be an investigation, and probably a court martial (assuming it was a military member who leaked the info). The trial would need to include motivations and other factors, but I can see situations where 5-10 years would be adequate, and maybe 25-50 (i.e. effectively life), but I would be very uncomfortable with life or capital punishment over this particular release. There are people from the cold war (Ames, Hanssen) who are vastly worse than this release, and they did it for money, not for any kind of "the truth must be released" motivation. They got Life, even though their information clearly resulted in the deaths of US affiliated agents, so I think that should be an absolute upper bound.