Very interesting. Man your best advice, keep your money in your pocket... until you have the hard part done, which is writing the song.
And your basic rules for a good song, understandable, interesting, memorable and convey feelings, I also agree with.
But for me this is when the real work begins... my insatiable fumbling for greatness.
My most useful instrument is my ear and after I have a song written and roughly arranged I put in my Ipod and I can listen to it for months waiting for it to speak to me, to give me
some hint of it's desired direction.
So I never consider a song complete. I recently learned to play
the F#min chord and completely rearranged an older song.
It didn't make the song great but it moved it along a bit.