Open-Closed Term Sets & Terms in SharePoint 2010

The 'Move Term Set' action for term store management, in combination with Restricted and Full permissions settings for the Managed Metadata Service, can be used to enforce the Open-Closed principle for adding new term sets to your taxonomy. Allow local taxonomy managers to add column specific term sets (open for extension) in local site-collections, but do not give them full permissions on the core term store (closed for modification). Then have a policy for periodically reviewing local site-collection term sets to incorporate useful new ones into the shared core term store.

Note that each term set also have an open-closed setting for controlling if new terms can be added to the term set or not. Users with restricted or full permissions are allowed to add new terms to open term sets, users with read permissions are not. Use the combination of open + restricted to enforce the "open for extension, closed for modification" policy for term set terms.

Note that access to the term store is granted per web-application using the app-pool account, not per user or group.  In addition, you can control permissions on each term group using the "contribute" and "manage" settings for granting rights to users and groups. If users are not granted contribute rights, they will be restricted to read even if the web-application's MMS connection permissions allows for more than just read.