Kair said:
There is no need to state something that we have already agreed on.
Quality in music is not irrelevant.
Music is the composition of melody, rhythm and timbre. Already here some of the poorer genres of sound fall through.
Polyphony is vital. With no polyphony, all you get is simple music.
Poetry is also used in combination with music, this adds to the quality of the music.
Variation is very important.
Pardon me, but I have not yet found myself agreeing with you.
I do not think that music would need melody, timbre or rhythm as they are but three aspects of what music can consist of. I take it you do not hold musical projects which only work on one aspect in very high esteem? There are a lot of artists who do not care for polyphony or other aspects of music - be it rhythm, timbre or texture, melody (artists purely concentrating on pitch also exist) which, I dare say, are not very simple in either structure or production wise. I am not clear if you base your idea of quality springing from a combination of melody, timbre and rhythm. Which brings me to hip-hop: while it might lack in variation of texture, a lot of the attention goes to rhythm and melody. If you can't spot them then chances are you are not paying attention.
Variation is important, I do give you that - but I find it doesn't mean variation between instruments, as there are whole worlds of variation within every sound and instrument, which can constitute into intricate pieces. Whole worlds outside those three elements you mentioned, and I think many people ignore them completely. I do think that there is a lot to learn from pop music, however, there is a lot more in other kinds of music.
While poetry can add interest I find it is only a trend of adding such into a musical piece.
Clarification on the quality argument: qualities can exist in sound, quality does not. Preference is only a perception of quality.
Minor edit: I have no clarity as to whether you do or do not listen or attribute "quality" to rap, techno, modern pop and so on - after re-reading your initial post, I have become confused.