I agree that a lot of the music rocks, but I think another important aspect of the music rocking is putting it together with the right mission. The slower, less discussed music is important for the hum-drum missions while in good campaigns, the climatic missions are really enhanced by the appropriate music.
Hate the Traitor for example, used the perfect music for the mission. But at the same time, that music is not very good for most missions, the ambiance/normal track specifically. The music helps to set the pace of any campaign.
My personal favourites from FS2 are numbers + leviticus, I also like the cinema track but it can only be used sparringly to have its full effect. I'm not sure why people ragg on the vocals, I love the vocals in FS2.
FS1 music seems to have more urgency, while FS2 seems to go for the 'this campaign is really epic and galactically important' slant.
EDIT - One problem with FS2 music was brought up by someone at an earlier time, the music has very little in the way of allied/enemy arrival cues. I believe that each FS1 track had its own (and multiple) enemy, allied arrivals + death tracks and perhaps even nrml tracks (whereas some FS2 music shares the same normal-track (normal/ambience as in the music played when there are no enemies around)).
