Kelly's point in "Writers need practice too" is so parallel to an example that I have been giving for years that I want to tack it on for good measure. "No one would expect a pianist to sit down for the first time at a piano and play Rachmaninoff's Piano Concerto No. 3. They have to start with "Chopsticks," and scales, and after a great deal of practice and effort, years and years of repetition and acquired skill, then and only then, can they approach something as complex and masterful as Rachmoninoff's Third. Why would you expect differently of a writer?"
The answer, of course, is the same one that causes business executives and grocery managers to brush aside edits of their letters and posters: The vast majority of people grow up learning to speak, and in some way or another, to write, in order to communicate, and most people begin to feel, at some point, that they have acheived a level of communication that is sufficient for any eventuality--generally when they get out of school. Thus, you have executives who try to write advertising copy and don't understnad why people just don't get what they're saying; grocers who can't understand that it matters whether the sign reads "Carrot's Sale Price" or "Carrots Sale Price."
One of the concepts by which this group was named was the term "wordsmith", which is wonderfully accurate in its depiction, suggesting persistance, effort, craft, and banging away at something until you get it just right. I agree with Kelly's implication that "talent" lies along a bell curve, with the exceedingly able and the utterly incapable being the minimal extremes, and with most of us lumped into that large, ambigous middle zone. I think Lyda's point is perfectly accurate once you hit that middle zone, once you chop off the two extremes of the curve, which I would (very casually) estimate at approximately 1-2% of the population. (There are other factors that I think further sub-divide the field--education, personality, etc.--but for now, I'm just talking about "talent.") For the rest of us, I think "talent" is mostly a useless term, independent of its accuracy/inaccuracy.
Borrowing from Steph Zvan, though, I would suggest that one dichotomy that plays a large role in aiding someone toward being a writer is not between "talent" or lack thereof, but between imagination and lack thereof, something that can only be fostered and nutured when we are children. By the time we hit adolescence, our basic brain patterns are established, and while change is possible, it is also unlikely, and requires a very large shift to force us out of those comfortable, predictable mental ruts.
For millenia, we have regarded creativity as a drive, a compulsion. I think compulsion is a strong element of it. Creativity is a type of insanity--focused, honed, practiced even--but a compulsion nonetheless, to find and build that which is not yet in existence. I think the word "drive" captures its essence, both the need to create, and the persistence with which we must create. We walk the line between the rational and the irrational, reaching out to either side and grabbing hold of what we find there, and tying together the things we grasp in each hand, linking the rational to the irrational both to find our balance as we walk that fine line and to push that frontier one way or the other, depending on our mood, our experiences, and who we define ourselves to be. And, like my argument here, we always face the potential of wandering off into the nonsensical, and must struggle to bring back from that precipice something of value: a metaphor, an insight, an FTL drive, a quantum entaglement of minds that yeilds telepathy, however inconvenient it may be.
I think that when we are children, our imaginations can either be encouraged or they can be shut down, and that simple approach taken by our parents, over which we have exceedingly little control ourselves, can do more to define our creative potential than almost anthing that we do as adults. That said, I am not yet ready to yeild my free will to the mores of psychological predetermination; reading, learning, speculating the whys and wherefores of the world--all are things that we can do to increase our opportunities to spark that drive, to jumpstart the engine of our minds, should we so desire. If, individually, we have no control over certain elements of our temperment, then those elements should not factor into our calculations of what we can do and what we should try to do. There is a difference between the impersonal hypothesis of "What makes a writer a writer?" and the highly personal "Can I write?" Yes, you can write, and if you feel so compelled, then you should, you must write, and nothing anyone says to you should interfere with that goal. In the end, this is all speculation. You must define your own success, and tell your own story.