Dating from WWII to the early 1980s, the documents "cover the gamut of foreign relations, intelligence activities, and military operations - with the exception of nuclear weapons data, which remain protected by Congress."
The reason for the delay? Recalcitrant spy agencies are unwilling to conform:
Some of the agencies have thrown up roadblocks to disclosure, engaged in turf battles over how documents should be evaluated, and have reviewed only a fraction of the material to determine whether releasing them would jeopardize national security.
In 1995 President Clinton first initiated the release of these documents, although President Bush amended it in 2003.
In anticipation of the missed deadline, President Obama has promised the agencies an extension past the deadline of an undetermined length.
Both the President and the agencies involved share some of blame for this. The President should not be giving open-ended extensions unless a good faith effort to clear the backlog is being made. Likewise, the spy agencies must realize that they are answerable to the people and the work done generations ago deserves to be open for all to see.
Releasing these documents is an important check on the power these agencies have and is a way America separates itself from third world dictatorships. Let's hope these agencies meet this requirement soon.