Manning's actions were unprecedented. His treatment is inhumane, but it's hard to know WTF to do with him or what to charge him with since it just hasn't happened before. They want to get it right the first time.
Daniel Ellsberg, a military-industrial-complex wonk, released Top-Secret documents on the conduct and strategy of the Vietnam War to the New York Times. He was the first U.S. citizen ever tried by the Federal government for publicly "leaking" documents for its own sake, as opposed to conducting espionage for a foreign agency. The long and eventual outcome concluded that the Times had the right to publish what it was freely handed, and Ellsberg was found innocent by reason of mistrial because the FBI repeatedly violated Fourth Amendment statutes trying to obtain evidence to use against him.
The scale and method may be unprecedented, but the legal quandry itself is only slightly complicated by the fact that Manning is a serving member of the military.
I appreciate the comparison, but there's a fundamental difference between Ellsberg's leak and Manning's leak. Ellsberg was releasing documents which were narrowly focused and consisted mostly of policy debates behind the scenes. He wasn't releasing the coordinates of every firebase in South Vietnam, or technical specs on US Air Force assets in-theater, he was releasing internal memos showing that a significant portion of the Pentagon upper brass knew the war was becoming unwinnable. Some of those discussions contained classified strategic info such as tonnage of ordnance dropped on various areas, and the fact that we'd expanded the war illegally into Laos and Cambodia, but those were germane to the discussion.
If Manning's leak had been restricted to a couple of incidents/thematic problems (such as the firing on civilians or airstrike on friendly forces), I would be in support. Those are failures of command that need to be brought to light and addressed. But he didn't stop there, nor did he use the judgement to weed through the data and find the truly "bad" things that needed to be brought to light. No, he took the lazy and dangerous route and just bulk-copied masses of data and handed it over to Assange, who also took the lazy route (albeit, it's partly because of the Wikileaks model that they don't edit what they're given) and released the entire data dump.
Frankly, it's akin to napalming the village to save the villagers, IMHO.
And of course, Ellsberg was a private citizen. Manning took an oath of allegiance and an oath of trust, and breached both.