Over time, my users have added many tags to issues that were later removed because they were unnecessary. These tags remain in the database forever and show up in the dropdown list of suggested tags.
This can be especially problematic if e.g. several variants of a tag exist (let's say two different spellings of the same word): User A might use tag A but user B might use tag B, leading to further tag fragmentation and making the tags useless.
I know I can delete tags one by one, but for that I have to click on every single tag, check if it's unused, and then delete it manually.

Here are two suggestions:
1) In the tag list, show the number of issues this tag has been assigned to.
2) Add a maintenance button to the tag list, allowing to automatically remove any unused tags (i.e. 0 assigned issues).