To have simplicity in music, whilst still retaining interest, I think is the ultimate goal. Music can of course be complicated and still be good, but the one does not necessarily follow the other.
I was thinking about this when listening to some of King Crimson's first album whilst round at Al's place. That album had a certain spark and energy of "the new" about it, and I could see why people liked it back in the day. Music hadn't really been done before like that.
However, I've also heard some quite revolting prog-rock bombast which aims to impress with it's alleged cleverness, but succeeds in only creating a stinking mire.
It's sometimes interesting to see what you can take out of a song, and still hold the listener's attention, rather than what you can add in.