In addition to providing Agency metadata and facilitating informed discovery of datasets, there may be a good role for Data.Gov to serve in also serving as a clearinghouse for discovering and facilitating access to datacentric agency web services, e.g. Open Geospatial Consortium Web Map Service (WMS) and Web Feature Service (WFS). In many instances, these types of services may lie scattered across and within agencies, with users accessing them via differing URLs, often with their own unique parameters, supported functionalities, versioning and quirks. This can be tremendously simplified by using Data.gov to aggregate these disparate services - an example of this can be found here: http://carboncloud.blogspot.com/2010/01/cubewerx-and-carbon-project-contribute.html - where CubeWerx and the Carbon Project developed a cascading OGC service to facilitate and simplify access to a variety of data services to support Haiti response and recovery.
Essentially, what this provides is a single point of entry, which provides a catalog of services collected dynamically from disparate OGC WMS and WFS servers, which then can all be accessed seamlessly from one point, as opposed to having to add multiple servers separately.