Thats a really tough poll to answer, because the answer is really "All of the above."
However, writing skill is a basic prerequisite. If the first installment of a Story Hour is just one long two page paragraph, I won't even bother reading a single sentence - I'll see it and back right out.
Competent writing allows all the other elements to be seen. After that, I think the character of both the setting and the PC's is what draws me in - it did in Sagiro's and Piratecat's Story Hours, thats for sure. Wulf's drew me in because of the PC characterizations, and the distinctive writing style that got Wulf's personality across so clearly.
I'm not generally too fond of Story Hours that delve into modules - I might want to play in those modules! For example, there's a Story Hour about Freeport going; I know that one group I play wth plans on going through those modules. That means that no matter how well written, I won't read that Story Hour.
I also like Story Hours that use the conventions of the game in neat ways and tht give me cool ideas to spring on my own players - Piratecat and Sagiro are particularly good at that.