Once more, just "tossing the idea out there", I'd like to suggest possibly allowing a post's upload date, age, or posts from a certain date range to be metatags. It would be interesting to to get posts from a certain date or age-range, and it would be helpful for blacklisting (which I believe should be a priority, too).
One scenario I feel this could be useful is for the augmentation of blacklisting: no matter how extensive of a blacklist one sets up, it's impossible to avoid those squatters who upload disgusting or grotesque content, and fail to tag or rate their posts properly.
In these circumstances, the uploader leaves his upload rated as questionable, and usually doesn't tag it at all. If we could add a blacklist rule along the lines of "blacklist all posts that are rated 'Q', are only tagged with 'tagme' AND are less than 30 minutes old", the chances of having to see that type of content can be greatly reduced. Because, by then, someone who has have no problem seeing that kind of content would have most likely seen it, tagged it, and rated it properly.
For the most part, blacklists work very well, but we really need to be able to use more metatags for blacklisting, so that we can more effectively avoid content presented in the aforementioned scenario (and others).
Basically, I'd like to see more metatags, and I feel that tag blacklisting is something that is very important, and should have a high priority.