I don't know shit but here goes my take.
First, while I do believe there is a duplicate content penalty, I think most people get too excited when they here about such things. I pretty much posted my feelings on it here
http://www.greenguysboard.com/board/...7&postcount=44
Content is also defined by more than just the text and images. Presentation also fits into content.
With date, category and tags you are sorting your posts in a manner that is actually beneficial to the surfer, Your allowing the information to be presented to the surfer in the manner he is most interested in. In other words you are enhancing the surfer experience. I find it only logical that Google et al would find that a positive thing rather than penalizing you for it.
Finally...damn I forgot what finally was. If I remember I guess I'll just post again.