I've been tracking the Rock Band sales rankings more or less daily for close to two years. My conclusion is that I can't predict sales rankings at all. It doesn't matter if I like a song. It doesn't matter if I think a song is good. (I can be honest. I like a lot of crap, and I can recognize that songs are good without necessarily liking them.) It doesn't matter if the forums are excited. It doesn't matter if the song or band seems to be generally popular. It doesn't matter if the song seems like it will have interesting gameplay. I just can't predict with any degree of accuracy.
It's fun to track the sales of songs, and be excited or surprised or disappointed when sales exceed or fail to live up to my expectations. But I don't look to sales in Rock Band for validation. I'm not wrong to like a song, just because it doesn't sell well.
Fundamentally, I don't understand people who get angry when a song does worse (or better) than they would like. Sales performance isn't a judgment of your taste in music, and even if it is, what do you care what other people think? You play music you like, I'll play music I like, and we'll all be happy.