The other day, I asked this question about re-tagging closed questions and, judging by the answers, my conclusion isn't that we should re-tag them, but that we should delete most of them.
Here are some interesting links that talk about deleting questions:
This second link says this:
Minor forms of bad behavior encourage worse ones: that a neighborhood with lots of graffiti and broken windows becomes one where robberies occur.
So, should we start deleting or old, closed questions? Or maybe only does who doesn't have answer?