User didn't know what the 'Tags' checkbox would do if unchecked.
When creating a vocabulary you're presented with a choice of 'Tags' as a checkbox. This controls both autocomplete and whether users can create new tags when posting content.
Implications:
Taxonomy form handling needs a complete rewrite.
Solution:
[#340652] (separates the widget and term creation).
We should remove the checkboxes for "Tags" and "multiple select" and replace this with a radio for:
Tags/autocomplete
single select
multiple select
