What makes a good writer?
Two things: one, the drive to actually write pages, without which you are just a talker; and
two, the ability to tell a compelling story.
Having a sense for how a story is supposed to go into your audience's brains is, I think, what it's all about.
This seems obvious but we've been looking through a lot of writing samples so we can hire some freelance South African writers. In the absence of people having proper spec tv scripts (West Wings, CSIs, etc.) here in SA, we asked people to write one act off an outline we'd written for ep 14, figuring, if it's not worth your while to write ten pages in order to get a writing gig, it probably takes you too long to write ten pages, and we can't use you anyway.
And some of the samples sort of look like the show, but there's no story telling going on -- even though we already give the people the outline. The guy we hired managed to solve some of the story problems inherent in the outline. We're actually using some of his ideas in a rewrite. I think that's a good sign...