I've entered this based on various comments made in the RM forum. I think this will cause trouble in a production environment. At the moment you can create custom metadata but there is no way to delete it, it cannot be done via the user interface not through editing the XML schema under the covers.
There needs to be a way to delete a custom metadata field, although there are issues about doing this in a production environment, for example if that property is being used for access control, its far too easy to make a mistake (e.g. spelling mistake, wrong datatype) when creating custom metadata - and then there is no way to delete the mistake.
Another approach to this might be to make the metadata property inactive, but doing this may then cause problems with duploicate property names.