I'd say the best thing to master is solving problems.
In the case of developers it may mean that you never become a master at programming, but what's more important: solving a problem or having elegant, well documented non-solutions?
Hackers make things work, and I think that's more important than being a "master" - I don't even like the word master, it almost feels egotistical.
Hackers (by my own definition) make this thing here work now.
A master of his art has the time to design not just his solution, but the task itself, and the experience to understand all the implications of a given solution.
In the case of developers it may mean that you never become a master at programming, but what's more important: solving a problem or having elegant, well documented non-solutions?
Hackers make things work, and I think that's more important than being a "master" - I don't even like the word master, it almost feels egotistical.