Yes, I wonder about that too. Greenwald seems to be very eager to tell us that Snowden has more than has been released. The implication is that the rest of the information is too dangerous to release.

Apart from the two immediate questions: "Does that information actually exist?" and "Why is it too dangerous?" We must ask ourselves why Snowden isn't forthcoming with some less sensitive proof to bolster his claims. It presumably would have been possible for him to have foreseen the reluctance news organizations had to publish the information. Why not have something more along the lines of a proof-of-concept to dispel the doubt that comes with incomplete information? Having a recording of some peoples' phone records would be just that kind of proof (provided he couldn't have reasonably gotten their records any other way.)