[dotancohen]

Your post really hits the mark for me. One of my favorite compositions is Beethoven's fifth, and I sometimes find myself identifying a new motif or combination of motifs in that work, even after decades of listening to it. It is a very rich piece, and the motifs develop slowly, simple at first but getting more complex in time. Not more complex motifs, mind you, but the same motif gains a few notes here and there, and sometimes shifts slightly. Very rewarding to find new ones, and only by reading your post do I discover that that piece challenges me actively, as opposed to more passive music.

The Wall and The Downward Spiral have similar elements of challenge, now that I think of it.