Apparently I can't have 2 ideas in 1 day. so, here it comes:
Tag purity
Instead of setting the purity for each tag explicitly it can be computed as follows:
purity index = ([some number between 0 and 1 that needs to be adjusted with some time] * [number of yellow pictures with said tag] + [number of red pictures with said tag]) / [number of all pictures with said tag]
now you can show a color by looking at the purity index number, like 0 to 0.3 green, 0.3 to 0.6 yellow and higher red.
there are of course more advance mathematical distributions that can be used with running averages so that the algorithm will only update the database with every few dozen or hundred submissions for most used tags and are O(number of new submissions since last calculation)
btw, With this you can make the purity filter a range slider which uses a weighted average of tag purity*log(tag usage).
so that if I can search real NSFW pictures that don't contain nip slips or cameltoes(no offense to cameltoe guys).
Albums
You can allow uploaders to put multiple related pictures in an album to make finding similar images easier without users having to literally stalk an uploader.
For example I shoot a photo of a BMW, a benz, and a Jeep with the same camera on the same spot and tag them with car, road, BMW | Benz | Jeep
Currently NO ONE will be able to find the 3 images using the search feature; The only viable option right now is going through what the uploader has uploaded which is a pain if the uploader has a lot of uploads.
btw, thanks for this great site, You're always #1 for me, even without these.