Polly,

My my, you sure opened up a can of worms here. smile
my opinion for what it's worth is there is many things that have to come together on a really good song but i think without a doubt you have to have a stellar vocal performance with the right kind of emotion conveyed to really SELL the song and make people want to hear it again and again. that is what i think is the root of the problem of unsuccessful singer songwriters.
Again-just my opinion,

john