Yes, playing an instrument or a sport are 2 prime examples that are physically easy to see the hours involved in practicing. A lot of intellectual pursuits would follow too like any complex math, engineering, or even medicine. Auto mechanics fits too if you're expecting the mechanic to be very good. Coming up with a number of 10,000 hours seems somewhat arbitrary, but it's probably a decent ballpark figure. The problem with that too involves the quality of practice/training involved in those hours. A violin player who has practiced things he/she could not previously play for 5,000 hours will outperform another player who has practiced the same songs over and over again for 10,000. The same can easily be said for athletics. 6,000 hard hours of practice will obviously reap more benefit than 10,000 hours of minimal effort. Simply put, experience doesn't equal skill level, but you can't become skilled without the hours of practice.
Not sure I understand the question, but basically the point of the hockey story is how being the eldest on a team impacts a player's development and compounds his advantages over time. The cut-off for Canadian hockey leagues is Jan 1 of a given year. So players born at the end of the year have a disadvantage since they're less physically developed. The point is proven by looking at birth dates of Canadian Junior National Teams about 75% of players are born Jan-April