There are some natural laws "physics" (more precisely, dynamics) with crowdsourced sites, and one of these is the inevitable buildup of a long tail of cruft. If you look at anything from the vote distribution to the distribution of high scores on the site you'll see that same pareto trend.
Tags are subject to that same dynamic. There are a few tags which are very popular and a tremendous tail of tags which are barely used.
Unused tags are deleted automatically, so I used to try to retag questions which utilized esoteric tags....but it really is like cleaning out the Aegean stables.
Raising the privilege threshold for tags may help, but based on experience I think:
- Better UX (labeling, cues, feedback and prompts) would be more effective than privilege adjustments
- That may reduce, but will not get rid of the problem.
Stackexchange has plenty to work on, but I think a higher priority than cleaning up the tag creation process is for them to do a better job with the question interface because a lot of community effort is wasted on moderating, flagging and deleting lousy questions.