Right, first of all, this is (kinda) my opinion and also focuses on commercial pop artists so don't @ me!
I have been fascinated with this topic for a LONG time and had many arguments/discussions with various musician friends about it. I've heard the standard points; 'X factor is shit', 'go to watch local bands who are GOOD', 'people who write their own songs are BETTER'.
In reality, I think these points are completely subjective and very generalised.
To me, there are five factors that make an artist successful:
4. musical ability