Today, I had a problem I never had before with SharePoint 2010, being with the Beta or even the RC.
My Managed Metadata Server was running and a proxy created. To that proxy, I connected a site and everything seemed to work correctly. Until I created a site column of type Managed Metadata. In the place where normally you can select the term store to use for the column, there was a message saying "The required feature is not enabled for this column type", which was quite strange as I checked several times and all the needed features were activated.
As most of the times, my friend google helped me. I found this post, not that new so it means that it is a know problem, not only on the RC.
Basically, according to this post, you have to activate a feature that has the ID 73EF14B1-13A9-416B-A9B5-ECECA2B0604C. But this feature is not present in the feature list from the user interface, so you have to run the stsadm command for your site.
This solved the problem, but I really wonder why it happened only this time....