Since MC supports DirectShow it can play pretty much everything if the appropriate DS filters are installed.
On a vanilla Windows installation the support is limited to what Windows offers: mainly WMV, MPEG1 and AVI containers if the used codec is one of the old and quite obsolete video codecs that come with Windows.
QuickTime and Real Media can be added by installing these players and support for the rest can be added with DS filters (e.g. by installing the CCCP filter pack or similar).
One thing that is rare among video players is a fully featured database. MC has probably the best available media database, which is fully functional with video files too. Things like unlimited custom tags, automatic video thumbnailing, custom video thumbnail from a screenshot, possibility to add custom cover art from file, searches, various view scheme options with thumbnail and details views, smartlists etc are not common. Windows Media Player has only a few of these features and the rest of the video players usually have only a simple playlist that shows the filename.
EDIT
You should also mention the multi zone possibility and detachable displays. For example, MC can run two or more videos simultaneously in separate Zones.