I started a similar thread to this around the same time last year. It's simple - you have to compile a tracklist of songs that have meant something to you over the past year. The songs don't necessarly have to have been written or released in 2004 - they just have to have held some personal significance to you over the last 12 months. The only rules are that you can have no more than 20 tracks and you can have no more than 2 tracks by the same artist/band.
Give it some thought and post your tracklists. I'm still trying to put mine together (and have been for the past 3 days!) so I'll post mine later.
