The main feature I wanted that Windows MCE (at least the XP version) didn't have was user-definable tags for video, which MC appears to support VERY thoroughly (and consistently across media types).
And MC can export its tags to XML. While that might not seem like a big deal, consider what would (will?) happen down-the-line after you've carefully tailored the tags for hundreds of videos and thousands of stills, and you decide to switch to a different media management software. How you gonna get the tags OUT of your old media management software? There's no guarantee that the new software can import the XML, but it's better to have your tags in a (text) XML file than the tags being held "hostage" in the old software, with no way to get them out!
(my day job is a computer systems consultant, and so I spend my days dealing with data migration issues)
Of course, I've only downloaded the eval for MC12, but it looks really impressive, and I'll be buying it (MC13) soon...
DadCat