For my money (all 73 pence of it) essential songwriter 'kit' includes:
1) a soul
2) an ability to connect to the experiences of that soul and distill them to maximum potency
3) an ability to perform point 2 in a manner fully understandable and accessible to all
4) through mastering the balance between revealing and hiding
5) and thereby allowing the 'viewer' to 'paint' their own 'Mona Lisa'
6) using not much more than a few blunt twigs and an ashtray
I think that practice improves, but that ultimately you're either Michael Angelo, or you're not.
You either feel or you don't.
Example of fine songwriting? In my opinion something like "Christmas card from a hooker in Minneapolis" by Tom Waits.
Narrative.
Imagery.
Humour.
Empathy.
Brevity.
Having said that I once 'wrote' a pretty good song by picking words randomly out of a hat.
Vodka generally improves my perception.
That and love.
Perhaps we can define a good song writes by good songs?
Any other suggestions??