I got an MP3 player about half a year ago. It has a storage volume of 256 MB, which means it will fit between 50 and 100 songs, depending on the filesize (I rip them at lower quality, which also means they are considerably smaller, but I also download files or receive them from other people).
I listen to it all day at work, or while riding the bus, waiting for lectures... anything where I'm not at home and I'm alone. I change the playlist about once per week on average, sometimes more often, sometimes less. One thing about my playlists is that the songs always fit together. Right now, I've got a list that is purely Maiden, after many weeks where the max number of Maiden songs was maybe five. It is emphasized on live songs (much from Beast Over Hammersmith and the 1998 Madrid bootleg) and recent stuff (many songs from BNW and DOD; everything from VXI, either live or studio). It also contains a few classics.
I never, ever throw together happy and melancholy songs, unless they fit together for some reason. The big problem is that I have limited control over the track order, and that I will listen to the list at least twice a day. So, the playlist must work in a random order and it must have replay value. So I don't include stuff I like to listen to once in a blue moon. That is why my MP3 player is usually crammed with nothing but my favourite music: Maiden, BLAZE, Bruce, Judas Priest, Deep Purple, Pink Floyd, Dio, etc. The consideration is: I must like it, else I'm torturing myself. When I'm at home, I will put on stuff I'm trying to get into or that I want to give a listen to because I haven't played it in a while. Not always, but often enough.
So this does not mean I'm not open-minded to other music when I am only listening to Maiden the whole day out, but it's cool consideration.
Now, to my appreciation of music. My basic rule is, if it does not give me something to make my mind going, it is not worth listening to. A guitar solo can already make my imagination go wild. For example, whenever I hear the instrumental section of "The Thin Line Between Love And Hate", I think of a lake in a mountainous landscape. Don't ask me why. But this sort of thing keeps going on in my mind.
That is also why I don't like a lot of music many people think I should like. For example, I am a big fan of Queen. No matter if it's the early hard rock days, the later pop days or anything else, I like almost everything in their discography (except for half the Hot Space album). So, people have recommended to me bands like Radiohead, Oasis, Weezer, Green Day and even Robbie Williams, with the argument "it's not that different". To me, it is. With Queen, I can lay back, close my eyes and let my mind going if I have the desire to. Bands like Weezer (Perunish for "overrated") sound just too much like they're trying to be modern, avantgarde and controversial, without reaching any sort of musical depth. Robbie Williams is an admittedly great singer, but his music is entertainment, nothing more. I don't listen to music for entertainment. I do that when I go to bars or something like that.
Oh well, this is going to go nowhere, so I'll better stop here.