There is one important thing: some of those hours should come before certain age. For many disciplines/fields it is under 18 and for almost all it is certainly 25.
In other words, if you start practicing basketball at age 37, you are not going to make it to NBA even if you put in 20000 hours of deliberate practice. While this principle seems obvious for largely physical pursuits, it holds true for purely mental fields, as well, be it chess, programming or piano playing.
