well it depends on your definition of "quality" right? i do think that even though music is a subjective experience, it's still possible to identify objective components to a song that contribute to its subjective enjoyment (or lack thereof)..
in my mind, the "quality" of a song involves the following:
1) does the song have strong sense of melody?
2) how much sense does the song make, harmonically?
3) how interesting is it rhythmically?
4) the interplay of components 1-3 -- How complex was it? How much effort was put into it? Did it achieve the composer's intent?
5) X-factor: did the artist try to evoke something in the song? how well did he/she convey it? how did i feel after listening to the song?
My favourite songs have all of the above elements. But not all of the above elements needs to be satisfied, or even be present, to qualify a song as one that i like. A song could just be a simple melody, or have a minimalistic rhythm, but still be good. or it could just be a 10-minute drum solo. or it could be a joke song that made me laugh because the artist knew what it took to make me laugh.
Bad Music, to me, is flat-out Bad because it violates one of the above elements. An otherwise good song can be utterly ruined by ANY ONE of the following:
- a repetitive, cheesy or boring melody.
- using chords/progressions that don't make sense.
- typical, uninspiring rhythm, or a beat that fails to adequately keep time
- all the melody/harmony/rhythm is there but there's no harmonic/rhythmic interplay between them at all
- maybe all the elements are technically there, but i just got the feeling that the artist over-composed the song. or the song, for all its technical prowess, was ultimately uninspiring.
so there you have it folks, the yardstick with which i dispense severe hatred towards tunes and people
at least, not for the moment.