In a SharePoint 2010 Enterprise Wiki I was unable to select any Categories; the Categories field was greyed out and the Browse button just didn’t work.
Opening the Library Settings, and selecting the Wiki Categories column I was presented with the error message: “The required feature is not enabled for this column type”.
I spend some time trying to figure this one out working my way around the Managed Metadata Service Application settings and properties as well as Site Collection and Site Features, and finally… I gave up!
Luckily Google didn’t and sent me to this page: http://social.technet.microsoft.com/Forums/en/sharepoint2010setup/thread/2de08757-a1df-44a4-bcbd-d1d9a5e4077f
It turns out enabling the Taxonomy feature on the Site Collection does the trick. Now, why didn’t I do that when troubleshooting this one myself… well, the Taxonomy Feature can not be enabled from within Central Administration…