Who are you writing for?

And, perhaps more importantly, why?

If you want to be a writer, you should know the answers to these questions. But don't ask me what answers are 'right' or 'wrong', because I don't know -- hell, nobody knows! As Rudyard Kipling wrote (in a passage I've used elsewhere, but is just as appropriate here): "There are nine-and-sixty ways / Of constructing tribal lays / And Every -- Single -- One -- Of -- Them -- Is -- Right!"

So why do these questions matter, you might ask? Because their answers tell you what sort of writer you want to become, what aspects of your writing you should focus on when you're honing your skills. Basically, the simple fact of having the answers, at all, is far more important than exactly which answers you happen to have. For instance, let's say John Doe wants to become the darling of the academic community, while Richard Roe just wants to sell enough copies to make a shitload of money. Which of that pair would get more benefit from fully mastering the intricacies of arcane grammatical minutia... and which would be better off learning how to engender a strong emotional response in his readers?

Of course, "academic respect" and "commercial success" are hardly the only goals a writer might strive for. But they're two of the most popular, and they do a pretty good job of illustrating how a writer's ultimate goal can influence which skills he should focus on first. Oh, and while I'm on the subject, you may want to check out Neal Stephenson's Slashdot interview; the whole thing is worth reading, but the second question in particular sheds light on the differences between "academic writers" and "commercial writers".

Anyway: Who's your audience? Why do you write? If you want to improve your writer-ly 'chops', the answers to those two questions will go a long way towards identifying what aspects of your writing you should work on.