Another topic we had discussed offline before, so I’m just logging this for posterity:
From experience I find it essential to have auto-completion when adding tags to an item.
Also, if a new tag is created that does not yet exist it should prompt the user to confirm (heck, if you want to be clever suggest similar tags first to avoid accidental redundancy).
I have seen it in other library apps as well as project management software, where tagging without these features inevitably leads to a huge mess where tags exist in with varying case, typos, hyphens, underscroes etc.
This in turn compromises the core task Yadle is trying to achieve which is finding stuff fast and reliably.
The mess I have seen with other apps in the past was so big that I actually asked my colleague here not to use Yadle tags at all for now.
Autodesk Shotgun has solved this nicely with simple auto-completion, which leads to a much tidier workflow and easier tagging (because you type less to do the right thing).
They also have a simple tag management page to facilitate clean ups and deletion of outdated tags (sometimes tags on ly make sense in context of a project and only clutter up the database afterward).