Having done a fair amount of reading on expertise (though it is not my research area), I can say the 10,000 hours figure for expertise attainment is arbitrary and quite problematic. Psychological research generally shuns these magic numbers because they oversimplify reality. I don't fault Gladwell for wanting to simplify the dizzying complexity of research in this area, but I do take issue with his tendency to make claims that are not warranted from the research, yet claiming support from the research. This is quite disingenuous for someone with such clout. More on Gladwell, later...
The 10,000 hours figure does not coincide with the cognitive psychology research into expertise. The rule of thumb is that 10 years of effortful practice is needed for expertise. I will stress that this rule of thumb (or average tendency) is not the same as a magic number because there is considerable variance in human behavior contributed by existing experience (e.g., expertise in another area) and natural ability, among other things. I have played saxophone for over 10 years, but I do not have the natural ability and have not put forth effortful practice enough to become an expert. Thus, I am still a dilettante after easily 10,000 hours playing and 10 years of practice. Others have become expert saxophonists in less time. That does not change that the average tendency is still about 10 years.
The figure of 10,000 hours does not match up with the 10 year standard supported by the research. Say someone devotes 40 hours a week to practice (as if it were a full time job) that comes out to 4.8 years. Devoting 30 hours a week comes out to 6.4 years. Neither of these are reasonably close to 10 years. I'm not saying that it is impossible to achieve expertise in less than 10 years, but it is the average amount of time needed.
Lest you think Mr. Gruber believes this figure: when I sent a version of this post, he replied with a quote from Merlin Mann: "If I were half as smart as Malcolm Gladwell, I'd already have statistics and a clever name for my theory that he's mostly full of shit." (from here).