The reason that the 10,000 hour does not exist are: (1) different fields require different amount of training and dedication, (2) it is difficult to measure what "mastery" is because it is so hard to quantify, (3) some fields are more competitive than others and therefore requires more practice to get really good, (4) the way that one learns and practice can have a significant effect on one's skill level, (5) at the highest level there is always a factor of talent and luck. Overall, the 10,000 hour rule just means that it takes a really really long time and dedication to be a world class at something. For some it may take only 1,000 hours while others will not reach it no matter how much they try. I think that this meme has gotten out of control and perhaps people have focused more on the long hours than the joy of improving or understanding what improvement means. It is not always better to spend more time on something and in some sense the feeling that we should be working all the time has lead people to become less productive.
I think the problem is more (6) deliberate practice is hard to define. Some people believe that just sitting and doing problem sets is deliberate practice. Some people believe that hacking on a side project is deliberate practice of programming skills. I don't think that any of those two examples would qualify as deliberate practice.
How would you define deliberate practice, then? I would agree that deliberate practice may be hard to define across all skills, but it could be defined for a specific skill.
For example in programing, I would consider problem sets and side projects with functions and goals that are aligned but still outside of one's normal scope of work to be deliberate practice. Working on Project Euler problem sets could be considered deliberate practice. They are problems that can be done in most any programming language, are known to be great for learning a new one, but generally fall outside the usual work of handling data, transforming content, etc.
I would consider Project Euler to be akin to practicing scales on a musical instrument: playing scales is not necessarily music in a "having popular appeal" sense, but it does enable new skills and depth of knowledge of the instrument.
Taking the Project Euler example, consistently doing the exercises, if they go in difficulty for each done, is deliberate practise in (probably) algorithm design, but does nothing for architecture. But redoing the same exercises over and over, always improving the design of it in each iteration (examples: reduce number of lines, apply DDD or hexagonal, etc, though the solutions in Project Euler probably not the best for this kind of practise) improves architecture skills.
The idea of deliberate practise is to always be improving in the tasks you are doing, usually by taking more and more difficult tasks in progression, but also analysing the past actions to identify where to improve. For example, I always liked to play tennis (and have many years of competitive table tennis behind me). Before I started working with a coach, my practises were mostly playing the game, maybe having a few swings against a wall when I didn't have a partner. When I started working with a coach, we practises my service over and over and over again, changing targets in the field, the kind of spin, and analysing most of the services to see what went right and wrong with my service. I can only say my skills improved at more than 10x the speed than when I was just having matches with other people.
I know many people who do stuff for ages now, but they aren't someone you would ask for advice, because they're so bad at it.
I know two photographs, one is mediocre and does this stuff for over 6 years now (professionally for money) and one is superb and does it for 2 years (for fun).
Most people who get involved with a specific thing never get to the practice part. They just read a bunch of stuff, do a bit of it and think they know everything...
Possibly linked to luck, but I would add another important element: inate ability.
No amount of practice - 10,000 hours, 20,000 hours, or every waking moment for the rest of my life, is going to give me Michael Phelps' shoulder size or ability to process lactic acid.
There may be a (7) having to do with the age at which you start. Something like a 10k h rule would correlate with those who started earlier. To have any practical chance of actually putting in 10k h on the violin, it sure helps to start at age six.
It also helps if formal practice is the same thing as having fun for you.
