The ultimate measure of a song comes down to whether or not it moves someone. If a song is going to move someone, it has to start with you. I have many principles that I apply when it comes to writing, but only one rule:
If a song doesn't move me, then it isn't a song.
Write music that is, first, meaningful and important to you. If the music doesn't touch you, then why should it touch anyone else?
As far as "worship" songs go, why would you offer Jesus a song that doesn't even touch you? Would a song move Jesus if it didn't even move you? Don't y0u think he's worth more than that?
I think it's the difference in Cain and Able. Able offered something that he valued himself, but Cain only offered what was obligatory. Cain fulfilled a requirement, but still never gave God what God really wanted. I don't think the object they offered was ever the issue. Though it may have symbolic significance, I don't think God liked the sheep better than the produce. It was what each offering meant to the giver that God was concerned about. God doesn't need the sheep or grain. He doesn't need your song to feel good about himself. He wants you. If you come with the song then God wants the song. Otherwise it's just an activity, a program... a ritual.