New Yorker:
Pilot tests of ThinThread proved almost too successful, according to a former intelligence expert who analyzed it. “It was nearly perfect,” the official says. “But it processed such a large amount of data that it picked up more Americans than the other systems.” Though ThinThread was intended to intercept foreign communications, it continued documenting signals when a trail crossed into the U.S. This was a big problem: federal law forbade the monitoring of domestic communications without a court warrant. And a warrant couldn’t be issued without probable cause and a known suspect. In order to comply with the law, Binney installed privacy controls and added an “anonymizing feature,” so that all American communications would be encrypted until a warrant was issued. The system would indicate when a pattern looked suspicious enough to justify a warrant.
But this was before 9/11, and the N.S.A.’s lawyers deemed ThinThread too invasive of Americans’ privacy. In addition, concerns were raised about whether the system would function on a huge scale, although preliminary tests had suggested that it would. In the fall of 2000, [General Michael Hayden, the director of the N.S.A.,] decided not to use ThinThread, largely because of his legal advisers’ concerns… .
I'm sure it discarded some things, but the basic technical control that ThinThread appeared to have that Trailblazer (and PRISM) lacked is cryptographic authorization controls.