I dont honestly think that there is a definitive answer, for me I love songs that appeal tune-wise to my ear... and meaningful words. The sad, emotive and passionate songs written from an obvious stimulus are often more powerful I have found, as they were meant.
I have written a few songs that people actually liked! With me I never toil over a song, songs just happen, often whilst driving the car alone, sometimes the words happen first and sometimes the music. Often they are as a result of story or event that meant something to me.
I dont think you should keep changing and changing a song, But a song needs to be sung-in till it feels comfortable. And if I aint comfortable with it, i wont share it! Its like playing a tune to people you that wouldn't want to hear yourself!
But hey, I haven't written that many songs and what do I know?
I'm just glad that there are such talented people out there who give me a chance to enjoy their creations by execising my vocal chords!
Keep writing out there!
Cheers, Lucy