Handwritten notes, lengthy printed essays, newspaper clippings, and occasional photographs were posted. Comments -- and even entire conversations -- were written in pen, pencil, and crayon in the margins of postings, or on napkins randomly tacked to the door. Some content was controversial, some was self-serving, but in general it was all interesting, in some manner or another. Content was expected to remain for a week or two before it was removed.
There was a tradition of the senior class stealing the door at the end of each year, leaving the junior class to find it. During my junior year the door was found but, alas, it mysteriously disappeared again. Strong suspicions pointed to the administration; they weren't pleased with the "image" the door presented, especially to parents of prospective students. Humbug.
It struck me that this was a primitive RealWorld model for a wiki. I'm not sure how far the model extends to Luther's own Wittenberg door, but it's interesting to ponder. -- anon.
Note that Martin Luther pinned his theses to the WittenbergDoor first, as a rather conservative move to discuss the subject with his colleagues.
See also the YaleWall.