SharePoint without limitations is not a SharePoint! I tried to implement a metadata navigation for more than a site collection. It should be a global navigation so it is possible to change it only one time and all sites in all site collections are effected.
First, I select the option Managed Navigation after I have activated the Publishing Feature.
After choosing the Term Set with my global navigation and clicking the OK button it ends up with the following error message:
“The selected term set is already used by another site: xy”
However, there is no limitation without a workaround otherwise, it is not SharePoint! Tobias Lekman wrote a short description of how achieve the goal of a Global Navigation managed in a Term Store in his article Limitations with managed metadata. He also mentioned a good article on how to set up a Managed Metadata Navigation. Another good summary of workarounds found on Radu Tut.
..:: I like SharePoint ::..