As Loach mentioned, SIPRNet, being only secret-level in classification, isn’t terribly sexy. Most of the Iraq/Afghanistan data Manning leaked is rather ho-hum (“we saw bad guys and shot at them” or “one of our trucks banged into a German truck, and we need a new bumper”). As a former colleague of mine once exclaimed, “you can’t do fucking intelligence work with just a Secret clearance.”
The DoS leaks are a bit more embarrassing.
Bottom line is this (IMHO) - Manning was entrusted to safeguard data that is defined as having the potential to cause “grave damage to national security if made publicly available.” Some things are classified as Secret just because they could betray troop locations (remember the Geraldo fiasco?). He violated that trust. One way or another, there is a very high risk that some other mother’s son (or daughter) will die as a result.
What surprises me is that Manning, as an Intelligence guy, probably had TS clearance. Yet none of that seems to have been leaked. I find this odd, because I don’t find it likely that this shitbird drew the line at the Secret level.