1. You passed high school English. This isn't so much a great attribute as it is a prerequisite. Of course, as with most good things, you can use it for evil, so if you passed high school English only because you'd taken it four times already and the teacher threatened to quit and take the school district down with her unless they relaxed the grading scale so you could pass just by knowing your own first name and how to spell it (reasonably well, at least), then perhaps you should make sure you can truthfully claim the next nine reasons before embarking on a writing career--or any career, come to think of it. (Yes, that was a run-on sentence. More on that later.)
2. You enjoy reading. When I was a young, inexperienced (read: pompous) aspiring writer, I scoffed at the advice that great writers were great readers.
"What?" I said with great incredulity and astonished amazement, "that's ridiculous!! How can I spend time reading and still have time to write??? Doesn't make any sense!!! Perhaps those who have time to read are just lazy people not suited to the higher art of writing, such as I." If you noticed how inane, over-written, childish, grammatically incorrect, and punctuation-deficient the above sentence was, you have promise. Great promise. And by the way, reading truly is one of the best tools for a writer. Not only do you get to do what you presumably love (why else would you write if not for others to read? That's why others write--so you can read), but you also learn a great deal from reading the work of others. Yes, it might be all about what not to do, but being taught how not to do something is still enlightening. And a good share of the time it will be a spot-on example of good writing. Take notice.
3. You have the need to express your thoughts with others. Yes, this can be accomplished by chatting up your co-workers at the water cooler or Facebooking your hundreds of "never-before-met-and-never-in-a-million-years-will-meet" friends, but those thoughts are not generally all that worthy. If, however, you have firmly held beliefs that are worthy and can share them convincingly with others, either through articles or books or short stories or poetry, who knows? You might just help to change the world one reader at a time.
4. You have the skills necessary to write well. Yes, I said skills, not talent, desire, or patience. Those come later. Skills require honing, which means studying, learning, practicing, and taking constructive criticism. Skills can be sharpened, skills can be learned, but skills cannot be neglected if you want to be a writer--a published one, at least. Nobody says it's fun, but it is necessary.
5. You have the talent to write. (See? Told you I'd address this later.) This gets a little sticky. You can have talent and not the skills and fall flat on your face, but because you can learn the skills, you're fine once you pay your dues and learn them (the skills, not the dues). However, once you learn the skills, if you still don't have the talent, your work will only go so far. So talent without skills isn't good, and skills without talent isn't all that great either, so it would be in your best interests to have both. And if you're not confused by this paragraph, then I haven't done my job.
6. You have the desire to write. Desire is worlds away from either talent or skills. Talent is more of a natural attribute, and skills can be learned. But desire... well, desire is that indescribable force that makes you want to write even when, logically, it makes no sense. It's that driving force that pushes you to your computer when you'd much rather get that extra hour of sleep or eat a meal at least once this week that you didn't have to scrape mold from and eat with your eyes closed or clean up that vomit trail the cat left on your rug during the night and you stepped on in your zeal to get to your computer to assuage your desire to write. It's what keeps you writing when the talent threatens to jump ship, the skills claim amnesia, and the vomit grows colder still.
7. You have the patience to write. Patience is not something I consider a natural attribute. Have you ever known a three-day-old baby who doesn't scream for its next meal, but instead lies there quietly awaiting its delivery? No, me neither. How about that five-year-old who's throwing a nuclear-powered tantrum in Walmart because you won't buy him a ball to join the other balls in his closet from your last 73 visits to Walmart? Is he patient? No. What about that jerk in the traffic jam who insists on blowing his horn even though he's at the top of a rise and can clearly see the cars and trucks lined up for seven miles ahead of him--oh, that was you? Sorry. Well, you can answer this then. Were you patient? No. See? Patience is not natural. Patience, like a skill set, needs to be learned, encouraged, cultivated.
Just a disclaimer here: learning patience takes patience. Weird, I know.
8. You're willing to obey the rules. The rules of writing have a valid purpose. Honestly, they do. Learn them, memorize them, use them. They're your friends.
9. You're willing to break the rules. That said, once you've learned the rules, figure out how and when it's okay to break them. Then do it. Yes, I said do it. It's okay, really. Just do it, darn it! You'll know if it doesn't work either through your own intuition as a schizophrenic, rule-obeying/rule-breaking writer, or your editor will give you a hint. (Remember the run-on sentence in #1? Yes, that breaks several rules of writing, but sometimes you just have to do it to make your point.)
Just a reminder, though: Gratuitous sex or violence? No. Non-gratuitous, well-written copy observing the rules of punctuation and grammar? Yes. Gratuitous words in an impossibly long sentence? Sometimes. (See? It's easy.)
10. You're willing to be humbled, thrilled, and awestruck. There's nothing like seeing your writing in print to make you feel all of the above. Humbled because deep down, none of us ever feel worthy of what we're capable of doing. (But you did it!) Thrilled because it's a joy you can't explain when you see your name on the front of that book or on that article or short story or poetry or essay. (Because you did it!) And finally, awestruck because you come face-to-face with the person you always wanted to be but deep down inside never thought you could be. (But you are.) Yes, you are.
You really, truly are.