I have compiled a brief set of reasons not to use Windows Media Player 9 - 11. Winamp and Foobar are good alternatives. I've not mentioned DRM here, but that is more than just a WMP problem. Anyway, here we go:
1. Inadequate CD ripping facilities.
By default, WMP rips into WMA at a poor bit-rate, so you have to remember to change this in it's settings. Also, by default, error-checking is disabled. Even with it enabled, WMP may be fast but it hasn't got the best encoder out there.
2. Album Covers
If you put your own high quality album covers as folder.jpg in an album's folder, it replaces it with its own poor quality image. It also hides folder.jpg files (marks them as system and hidden files). To prevent this you have to disable it from retrieving album covers and managing your music library.
3. WMP messes with tags and filenames
Firstly when you rip an album with WMP, you have to individually edit the album/artist tags for each song and editing the playlist is a pain. There's also no overall playlist time. There's also little annoyances like it uses 'And' and 'Of' instead of 'and' and 'of' in tags, although these are down to personal taste.
4. Overall 'Bloatedness'
Media Player 11 uses around a 100mb of memory just to play a track! (Itunes also uses similar amounts of memory too) It just tries to do too much being a video and audio player and doing both poorly. Also, I don't like to allocate the huge amount of disc space it requires, just for a media player.
WMP has poor support for playing variable bitrate mp3s, displaying the wrong average bitrate and seek time. Some other software and hardware players have the same problems.