After reading the March 2003 memo ( NYT and WP ), I feel like the youngest kid at Passover dinner, who by tradition asks the question "How is this night different from all other nights?" Except that in this case, I'm left with the question of "How is this torture memo different from all the others we've read so far?"
I agree with Marty that the answer has to do partly with the bureaucratic manner it which it was conceived, authored, published, and classified. It's highly unusual for such a broad statement of administration policy to be issued over the signature of a deputy assistant attorney general—and that such a low-ranking official would basically be speaking for the Justice Department and the White House on these issues through the interagency process to the Pentagon. So, this memo is different to the extent that it didn't come from Alberto Gonzales or Jay Bybee or someone else of significant rank.
It's also different because it appears to have been conceived entirely by the super-secret-squirrel (a military doctrinal term) working group of lawyers that included David Addington , John Yoo, and a handful of others. That cloistered environment facilitated much of the legal reasoning in the memo; it also ensured it would be highly classified and kept from public view for a long time.
But what about the legal reasoning? Is this really any different from other memos we've seen (and written about) so far? It's certainly longer. And as Orin points out , it alternates between solid and shaky analysis. But in general, I think Emily's right that this reflects the same broad, sweeping rhetoric we have seen before in other torture memoranda hatched in the White House, Justice Department, and Pentagon. What makes this memo significant, I think, is the way that Defense Department (and other government agency) personnel relied on this memo to create the detention and interrogation regime at Guantanamo, Abu Ghraib, and Bagram Air Base.