Guest, how do you know who writes songs and who doesn't?   

And Ron's right about the maître d' and other consumers. You don't have to be Michael Jordan or Phil Jackson to tell when somebody's made (or missed) a 3-pointer.   And you don't have to be Tom Paxton to recognize a good song.   
In some ways I think the "laymen's" opinion of a song counts for more than the "craftsmen's."   The latter may admire the handiwork that they, with their specialized knowledge, can detect, but that may color their perception of the finished work.

I'd also add that every time I've consulted successful songwriters about their process, I get at least as many answers as there are songwriters.

I do find all the suggestions in this thread interesting and potentially useful.   We can use what applies to us and not worry about what doesn't.