Um.. I personally would really like a consistent look with the rest of the windows. Generally the media player looks unpolished and rough. The main playback interface, for example: i really hate the "3d" buttons. Reminds me of an old cassette player

. And the playback controls bar is too big.
I know the UI might not be the most important thing for you guys but I just can't help but notice it. I'd prefer a clean, simple and smooth design over the current one.
I do like the location and the look of the playlist, but since everything else looks too big, the general appearance looks kind of rough and bloated

.
Stick with a consistent design and don't make too much new bars for new buttons. I would even like the design if everything looks similar to the playlist (buttons and frames etc.)
Also, a media library would be a good thing to have: If I really would want to use it as a primary media player, I couldn't live without the library management system. Make it simple, yet functional enough for most users.
When i start up the player, the title bar shows: 23:59:59 - 23:59:59
It takes a while to open a video file, more than on most players.
There's a minor UI glitch when hovering the "save playlist" button in the playlist window.
When last.fm track changes (I haven't tried local files yet), the visualization stops and resets and the album art and title window looks laggy for a while. I'd like it to fade in/out smoothly.
Filetype icons are ugly but that can be ćhanged in the future

Last.fm playback is nice. especially the fact that you can see tracks in the playlist. Plus seeking (wtf). You could do the last.fm search bar a little smaller and stick it to a corner. And display album art and artist description in the center, like in the default last.fm player.
I really like the visualizations but I don't really use them usually.
But i might cause they are AWESOME. all that lines and stuff... You could take some ideas for the UI from there, although I'm not sure that everybody would like it. But it would give your player some character