I've noticed that the option for users to enter tag keys on a thread has disappeared. The option appeared for a while, and it looks like users were keen to take advantage of it, but as of a couple days ago it's gone.
Threads that had tags entered still have the option.
The forum listing shows which threads have tags attached.
The ability to search by tag still appears on the advanced search page.
Hey Bill - users should not be able to enter tags, this ability for users was removed based on some "unsavoury" comments made in tags, and we have no feasible way for them to be reported to moderators and edited.