Mark Pilgrim notes that "now we have thousands of webloggers who read other webloggers every day, and who themselves write every day, and they're not getting any better at writing. [...] there is obviously a secret third ingredient required for becoming a good writer. You need to read every day... and write every day... and X. But I don't know what X is..."
Talent + effort = quality: I suggest that X is having talent (some have other ideas). People who have a talent for something find it enjoyable, and do it a lot, and so get better at it. Child piano prodigies just love playing the piano, so they do it obsessively, usually instead of obsessively riding skateboards or reading comic books or creating cool tattoo designs inside the back covers of their Grade 9 notebooks.
Yet some people's talents probably lay dormant too, because they never discover them. There could be some Mongolian yak herders who would be great computer programmers, but no one may ever know.
Effort alone won't do: There are people without talent who do the same things, sometimes also obsessively. There are a lot of skateboarders who fall down all the time, and dedicated piano students who can merely plunk out chords, and aspiring tattoo designers who couldn't draw a convincing set of bloody dripping eyeballs held in a skeletal hand if you gave them a million dollars. They may enjoy what they do, and may work hard, but without the talent, they won't get that much better.
The guitarist/organist in my band used to be my roommate. When we started playing together, I had taken four years of classical guitar lessons, and he had just picked up the instrument for fun. He got way better than me in a few months—meanwhile, I discovered that I could play the drums half-decently without ever having tried.
I'm a pretty good writer, and perhaps a better editor. That's not my own assessment, but what my clients tell me. They often thank me for doing things with their words that I find both dead easy and fast, but which they either can't do, or couldn't without a lot of unpleasant effort. I have a talent there—not a great one, but enough to make a living—but I don't know where it came from.
While I write every day, and I read a lot, that's not the source of my ability to work with words. That's been there, in some way, since I learned to read and write. Yes, I work hard at it. But it's never been a chore for me to do well, and in that I'm simply lucky.
I'm also lucky to have a talent that can bring me income. And while I can drum, I don't dance very well, and I can't paint a picture. Even web design is not an easy fit for me—I might have a bit of an aptitude, but not a real talent.
So if you like to write (or tinker with cars, or sing opera, or ski, or translate Greek, or whatever) and lack the talent for it, that doesn't stop you from doing it. You're just not likely to get paid for it.