So, what things about music did you used to think but now you look back and say that they were really really stupid? I don't mean "I used to hate ____, but now they're my favorite band". I mean opinions about the actual music, not just certain bands.
The first thing that comes to mind for me is that I used to think that if a song was written in an odd time signature that the band was composed of horrible musicians. I also believed that any dissonance in music was absolutely horrendous and that if you compose music like that, you really have no idea how to compose in the first place. Of course, now I pretty much think the opposite. I love dissonance and odd time signatures.
Another misconception I had was that death metal was just people banging on their instruments and making random noises into a microphone.