Awesome post! I would also agree on melody being one of, or the most important part of a song. It's the hook or whatever you want to call it that makes a song impressionable for a listener. If I might add I think the idea of space, and "letting the song breathe" is also important. I find with most songs, especially if there is a very 'talented' guitarist is writing the song, that it is filled to the brim with sound. It might be a good option to some to have 'space' for a few bars, to add dynamic and space in a song.