I think its mainly because my PC collection is all sorted into folders, and my MP3 player oly does ID3 track listings. I had to rename everything and also, tried faking a folder structure which has kind of worked, but I shouldn't have to do that.
Another important aspect of any MP3 playeris the ability to skip tracks, and skip well. That means, being able to skip tracks, remembering what its played previously, and also skip a large region so that it doesn't play the same kind of track all the time.
Mine can't do that very well, and so, it tends to repeat tracks alot when you turn off the device and back on again.
Another important feature I think, is the ability to skip by certain attributes based on ID3 tags. Rating, etc.
MOOD:
I'd love one to catorgarise by mood also. An Added ID3 tag prehaps, that would ask you when you turn on the player how you are feeling, and play music you may not skip as often.
Once you tell it how you feel when you first turn it on, songs that you skip would be recorded in another Mood, and ones you don't would be recorded into the mood its on when you selected your mood.
It'd then have to work like this for a while until it got all of them, though of course, it's possible a track could be listed in more than one mood setting.