As described in Metadata navigation overview (SharePoint Server 2010), metadata navigation  includes the following user controls:

• Navigation hierarchies   Use and expand the capabilities of list views to navigate hierarchies of folders, content types, choice fields, or managed metadata term sets. This allows users to use list views to filter on a metadata hierarchy just like navigating folders
• Key Filters   This control appears below the site hierarchy control and can consist of several fields such as date, choice, content type, single and multi-value fields, currency, yes / no, and user fields. Any number of key filters can be applied in combination with a selected navigation hierarchy.

Navigation hierarchies and KeyFilter user controls registration  and also rendering templates for KeyFilter control are defined in MetadataNavTree.ascx and MetadataNavKeyFilters.ascx files.

Our goal here to show possible ways on how  KeyFilters control could be customized from the UI perspective.

Below is represented diagram on how KeyFilters control is rendered.

Depending on field type different rendering templates for composite field are applied as shown in table below.

But KeyFilter control has some limited functionality regarding customizing List Field Iterator  and composite field templates, for example it is not possible to specify alternative templates for composite field templates.

One of the possible ways for making KeyFilters control UI customization more flexible is to extend it by allowing to specify custom templates  for composite fields.

Project with custom Key Filters control implementation  is located here.

Resulting page with custom KeyFilters control and custom rendering templates applied  is shown below. In this example control field label and value in composite fields rendering templates are rendered  in one row mode.

KeyFilters Control

References

Source code