Single-use tags are pruned after six months. Sometimes, this removes spurious tags. Sometimes, this removes legitimate tags.
After a quick look on Unix SE, I see plenty of legitimate tags that are at risk. I would like a way to declare that some tags are legitimate and should never be pruned. This could be a 10k tool, for instance.
I realize this might not scale on Stack Overflow, but it would work for smaller sites (I'm using Unix as an example because it's the one I know best). A warning when tags are about to expire would be a helpful complement.