According to MSDN (link dead), the command line parameters that available for Windows Media Player include (with syntax and the changed behavior):
“path\filename” (e.g. wmplayer “c:\filename.wma”) – Start the Player and play the file.
“path\filename” /fullscreen (e.g.: wmplayer “c:\filename.wmv” /fullscreen) – Play the specified file in full-screen mode. The path and file name of the content to play must be specified.
/Device:{DVD|AudioCD} (e.g.: wmplayer /device:audio CD) – Play a DVD or audio CD.
“path\filename”?WMPSkin=skin name (e.g.: wmplayer “c:\filename.wma”?wmpskin=headspace) – Open the Player, applying the specified skin.
/Service:keyname – Open the Player showing the online store specified by keyname. (WMP10 and above only).
/Task NowPlaying – Open the Player in the Now Playing feature.
/Task MediaGuide – Open the Player in the Media Guide feature (current active online store in Windows Media Player).
/Task CDAudio – Open the Player in the Copy from CD feature (Rip feature in Windows Media Player).
/Task CDWrite – Open the Player in the Burn feature. (Requires Windows Media Player 10 or 11)
/Task MediaLibrary – Open the Player in the Media Library feature in Windows Media Player.
/Task RadioTuner – Open the Player in the Radio Tuner feature (current active online store in Windows Media Player).
/Task PortableDevice – Open the Player in the Copy to CD or Device feature (Sync feature in Windows Media Player).
/Task Services /Service servicename – Open the Player in the Premium Services feature, showing the service specified by the servicename parameter. This value is the unique name for the service. If the specified service has not been previously viewed, the servicename parameter is ignored. (Opens the specified online store in Windows Media Player)
/Task ServiceTaskX – Open the Player in the online store service task pane specified by X. For example, /Task ServiceTask1 opens the Player in the first online store service task pane.
/Task SkinViewer – Open the Player in the Skin Chooser feature.
/Playlist PlaylistName – Open the Player and play the specified playlist.