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Behind the screens
The first technical difference between iTunes and WMP is the way the library is managed. In iTunes you drag files into the library to add them whenever you acquire new files. This is a pain because you lose all song-related data like play count and stars and playlists if you want te rearrange the files on your HDD - you have to remove them from iTunes and add them again each time. In WMP, you specify folders for WMP to monitor, and all music in those folders is automatically in your library. This actually works pretty well, and somehow it manages to keep track of files even if I move them around while WMP is closed (I know it's actually keeping track of the files and not just removing/adding the file each time, because details like the play count are kept intact and the file itself is not modified to store this information).
The second technical difference is that WMP unfortunately doesn't support the AAC music format. That sucks because I rip most of my music as AAC, since it's an MPEG standard format. I expected that any decent music player would support it, but sadly not WMP. You can use third-party tools to add AAC support to WMP (meta-data support and codecs) but it's not perfect: the progress slider doesn't work for AAC files. (these codecs might fix the problem but it's commercial software) By the way, I don't think you can simplify the comparison to WMP = MP3 + WMA formats, and iTunes = MP3 + AAC formats, since a) iTunes can import WMA files by converting them, and b) AAC is standard and WMA is proprietary.
Using iTunes as a baseline, there are some positives and negatives to WMP's user interface.
WMP doesn't have iTunes' 3-pane browser that sorts by genre, artist, and album simultaneously. That means you have to flip between different screens to browse by those different categories. That might not be such a nuisance, except that each time you change screen, it resets your location in all the others, so you have to scroll down through your entire library to get back to where you were each time. It also resets the column by which you've sorted that category, which is really annoying. As a plus, though, WMP allows you to navigate the library in many more ways than iTunes (which would be nice, if only it remembered my preferences! grrr...)
Playlist management likewise has its ups and downs. Instead of Party Shuffle, WMP has Now Playing, which works slightly differently. Everything you play automatically goes through Now Playing - generally every song that's currently showing in the pane you double-click in is added to Now Playing, as opposed to iTunes playing either the library or a playlist or Party Shuffle. That's pretty neat, because you can browse to an album, or do a text search, double click a song and all of those will go into Now Playing and be saved there, even if you then browse away or do a different text search. Another neat feature in WMP is the playlist pane, which can be set to show either a playlist or Now Playing, and sits to the right of the main view. That makes it easy to drag music over to queue it up or reorganise the play order, without leaving the main library view. The downside of WMP's playlist management is that I can't find any way to mimic iTunes' Party Shuffle ability to automatically and randomly draw a certain number of songs from a specified playlist. In WMP you have to manually make sure there's always music queued up.
Ordinarily performance in programs like these, running on modern computers, shouldn't matter. Unfortunately, the Windows port of iTunes 6 was slow, and iTunes 7 is just terrible. Browsing WMP, in comparison, feels like casting off shackles and breathing freely once more.
I'll still use iTunes, however that's mainly due to where WMP foolishly stumbles rather than to where iTunes shines. WMP's inability to remember my viewing preferences and poor handling of AAC is terrible. Maybe in the next version WMP will come out ahead.