Jim Collins, in his book Good to Great, talks about what he calls the "Hedgehog Concept."
He devotes an entire chapter to this subject, in great detail, and even though I will not do it justice here, I will do my best to simplify it in terms of my writing.
Picture three interlocking circles, like the Olympic rings except there are three instead of five. The first circle represents what you are passionate about. What drives you, what is your "why." The next circle is what is one thing--just one--that you could potentially be the best in the world. Align this with your passion, of course. Lastly, you need to be able to make money at it. There has to be an economic value attached to it.
Me, I am passionate about writing. I want to be one of the best in the world (these are big shoes to fill, but even if I never do it--because it's hard to compete with heavyweights like Charles Dickens and Shakespeare, and of course there's Stephen King and J. K. Rowling to contend with too--I can say I gave it one hell of a shot). Lastly, one can make a great deal of money doing it.
Take Tiger Woods. He is passionate about golf. He is one of the best in the world. And, he has made a great deal of money doing it.
Okay, take John Q. Publish (second-cousin to John Q. Public). He loves to sit on his ass, all day long, and watch TV. One could say he's passionate about it--most would call it laziness. Best in the world? Let's not even go there, because he can't make any money doing it. Now, if we were to turn this on its head and say he's passionate about designing websites for people, now we can turn the hedgehog concept into reality for this one.
Do the one thing you're passionate about, drive yourself to perform with excellence, and you'll be successful. You'll be . . . great.