There are many factors that contribute to good music design, but I always come back to this: How does the album art complement the music? Is it a proper representation of the music? Does it make me want to pick up the album... or better yet, listen to it? There is really no formula; I just think it should be done well. If it is, it should give the listener a reason to remember it.
When one talks about album-art design, one could not possibly omit Invisible Creature, Inc., which has received four Grammy nominations for music packaging and has worked with all the big names such as Sony, Universal, Warner Brothers, Nickelodeon and MTV.