Synchronicity.
Just today I finished rewriting (recreating, rather) that old article with that idea in mind. Its funny to see what I wrote before ... and how much I really need a good editor.
I totally agree there is a lot of variation in how quickly people improve, but I'd sum up the difference as differences in attitude/approach. We could practice 2+2=4 for 50,000 hours and never get any better at it because we've already figured out how and why it works. There's nothing to improve. People that simply repeat tasks don't improve much, in my experience. However, if you approach practicing as figuring out time - e.g. how can you figure out this technique - then you can improve very quickly indeed.
Another major reason people don't advance as quickly as others is evident in this cartoon
This cartoon describes me for more years and I would like to admit. And like the kid in the cartoon, I have suffered frustration and poor performance as a result that I could have easily avoided, had I been payng attention to what I should have been paying attention to. C'est la vie, non?
Overall, I still think Gladwell's arguments about talent still hold even with variation in the time it takes to become a Master. Uber-learners such as Josh Waitzkin and the 4 Hour guy say the same thing. its not genetics, its approach. Maybe this is what you mean when you say personality, except that you can learn it like you learn anything else.
