Interesting thread, and quite often I have struggled to find an answer to this question. Lately, after numerous reads, I have come to realize that there are only few factors that determine one as a good writer. Although I agree that it requires an above average skill to be a good writer, but believe me, it is not a rocket science. Here are few factors that affect writing as I have come to discover:

1. Observation - Every piece of writing depends on how observant one is, observation is also synonymous to wisdom, the degree of knowledge one posesses and how specific details one can provide.

For example, this is how an ordinary writer writes:

I rode my motorcycle to Jawalakhel this morning, it was cold

As compared to a seasoned writer who narrates:

I accelerated my newly purchased Kawasaki against the autumn breeze which  increasingly became cold as I rode towards Jawalakhel.

2. Humor and articulation - If you are reticent in reality, you often miss out on humor you could have created (although I have come across people who seem tacit at times yet come out with good one-liners and repartees).

An example of an ordinary humor - She farted so loud everyone started laughing.

An example of a humorous narration - Thanks to the spicy asian food, her bile juices churned up the tikka masala emanating a pungent smell out of her rear orifice to everyone's agony.

3. Verbiage and vocabulary - If you are not good with synonyms, don't even bother writing and if you are extremely good with it, use it appropriately.

For example - A two year old yelled at his sister 'You are incorrigible'. It makes very little sense that a two year old would muster enough knowledge to use the world incorrigible.

Another example - I was so irritated by my neighbor singing loud, I almost called up cops against I was debating with my wisdom to dial 911 after having been tormented by my neighbor's out of sync melodies.

There might be more factors that enhance good writing, but this is what I came up with at the moment, but then again, what do I know?