"One thing in your song should always be on fire."
I will be the first to argue that good church music can be old or new, classical or pop or folk or whatever, content dense or repetitive, celebratory or somber. And as I've pointed out before, the best critiques of praise-band music usually come from within that world, not from outside haters.
Anyway, this "How to write a worship song (in five minutes or less)" tutorial from Blimey Cow is a heap of fun.