2. Snowden Lacks Stature and Insider Ties. Ellsberg had stature when he leaked the Pentagon Papers. As the Washington Post put it, "Ellsberg was a senior military analyst working at the Pentagon who had a direct role in drafting the Pentagon Papers." Meanwhile Snowden was, according to the Post, "a contractor who moved through a series of low-ranking jobs for the CIA and the NSA."

Ellsberg was also deeply embedded in not just the Washington establishment but the national elite, having attended Michigan's prestigious Cranbrook School (Mitt Romney's high school, you may recall from campaign 2012), Harvard University, Cambridge University, a Marine Officers training program (followed by three years in the military in command positions), and was later a fellow at the prestigious Harvard Society of Fellows, as well as the recipient of a Ph.D. in economics from the university. At the Pentagon, he helped draft plans for the conduct of the Vietnam War, which he would later see up close working out of the U.S. Embassy in Saigon, and worked on the Pentagon study of the history of the conflict in Vietnam published by the Times as the Pentagon Papers.

All of this meant that when he leaked the documents in 1971, Ellsberg had a thick web of social and professional relationships in the halls of power that helped shape perceptions of him and his actions, as well as a sophisticated historical understanding of what his act of civil disobedience meant and the political tradition in which he was acting. When he turned against the war, it was as a powerful insider joining his conscience to an existing upswell in public opinion.

Snowden, as a 29-year-old high-school drop-out with a GED who washed out of the military during training (he says, though no one has yet found evidence of this) and who spent much of his career overseas or off the U.S. mainland, has none of this -- no ties to the building of the programs he revealed, no ties in Washington, no pre-existing public presence on the American scene, no elite web of contacts and relationships. His turn against the state is the act of an outsider whose allegiances and personality are known to the media only through a handful of interviews.

3. Snowden Is Culturally Isolated. Ellsberg's actions came at a time when there was a robust social movement demanding change in the exact direction his revelations suggested U.S. policy go -- out of Vietnam. Without the anti-Vietnam War movement, it's arguable he would not have been as important a historical figure, or as daring.

There is no comparable movement to support Snowden, no major anti-surveillance marches on Washington or roiling college campuses, no public burning of Facebook logins and passwords. While there is a robust online libertarian movement concerned with surveillance and privacy issues, there is no force in American life at the present time arguing for change on this front with anything near the power and reach of the anti-Vietnam War movement.