Quantity is important, but quality is important as well. If I practice my karate 1,000 hours and another man in Alaska practices 1,000 hours of karate as well BUT he has received higher quality instruction, he might be practicing effectively twice as much (measured in quality, which is hard to quantify..haha)

10,000 hours of practice seems like a reasonable gauge of mastery. Its also how you define mastery. Mastery is defined by the people who came before who set the standard. If athletes' top running speed reached 3MPH and I came along and broke the record at 5MPH, I would seem like a master.