SharePoint 2010 Navigation Hierarchies and Key Filters
Navigation Hierarchies
Navigation Hierarchies appear in the left hand pane of the new user interface and allow users to navigate metadata taxonomy in order to filter the view of lists / document libraries etc. For example, I've setup a document library that contains documents with different security classifications. To do this I've added a column called Classification that has possible values of PUBLIC, CONFIDENTIAL and INTERNAL. Here is the document library:

I can turn on the navigation hierarchy for this library by going into the Document Library settings, selecting Metadata Navigation Settings, and then adding the Classification column to the list of Selected Hierarchy Fields.
Once this Setting is in place we can go back to our document linrary and we will see Classification as a available field in the Hierarchy:
By clicking on the CONFIDENTIAL value in the hierarchy, I will see only the CONFIDENTIAL documents in my library.
Maybe not that impressive in a document library that only has 15 documents, but in a library that may contain a few hundred documents this would really help users filter down to specific documents they want to find and work with.
Key Filters
Key Filters also appear in the left hand pane of the user interface just below the navigation hierarchy. Key Filters operate in a similar fashion to the navigation hierarchy. They are also used to filter a list or document library. The difference is that the user does not have to navigate a hierarchy, they can simply type in the value they wish to filter on, or in some cases select the value from a drop down list. When the user clicks Apply, their list or library will be filtered based on the value.

In order to configure the Key Filters option, the administrator goes into the Document Settings, Metadata Navigation Settings, and then adds the Classification column to the Selected Key Filters Fields.

These features should help your users get more value out of the metadata. If you are interested in using metadata for SharePoint security see the Metadata Security blog.
Navigation Hierarchies appear in the left hand pane of the new user interface and allow users to navigate metadata taxonomy in order to filter the view of lists / document libraries etc. For example, I've setup a document library that contains documents with different security classifications. To do this I've added a column called Classification that has possible values of PUBLIC, CONFIDENTIAL and INTERNAL. Here is the document library:
I can turn on the navigation hierarchy for this library by going into the Document Library settings, selecting Metadata Navigation Settings, and then adding the Classification column to the list of Selected Hierarchy Fields.
Once this Setting is in place we can go back to our document linrary and we will see Classification as a available field in the Hierarchy:
By clicking on the CONFIDENTIAL value in the hierarchy, I will see only the CONFIDENTIAL documents in my library.
Maybe not that impressive in a document library that only has 15 documents, but in a library that may contain a few hundred documents this would really help users filter down to specific documents they want to find and work with.
Key Filters
Key Filters also appear in the left hand pane of the user interface just below the navigation hierarchy. Key Filters operate in a similar fashion to the navigation hierarchy. They are also used to filter a list or document library. The difference is that the user does not have to navigate a hierarchy, they can simply type in the value they wish to filter on, or in some cases select the value from a drop down list. When the user clicks Apply, their list or library will be filtered based on the value.
In order to configure the Key Filters option, the administrator goes into the Document Settings, Metadata Navigation Settings, and then adds the Classification column to the Selected Key Filters Fields.
These features should help your users get more value out of the metadata. If you are interested in using metadata for SharePoint security see the Metadata Security blog.