I don't think you really need to master anything to be a photographer, or to take good pictures. Obviously knowing the technical stuff is really important, but mastery of it, isn't a requisite to producing amazing photographs.
There are some virtues (not necessarily disciplines) that have helped me:
- Patience: taking a photograph requires you to sit back and think. You don't always have that luxury, of course, but when you do, use it.
- Self Critique: most of us will be our harshest critics, but we need to temper that in order to effectively improve.
- Perspective: trying to look at the world differently, will translate to your photographs.
Outside of these, I would say the one discipline you should master is your job. Money helps heaps in photography. It's expensive. You can absolutely be an amazing photographer with an inexpensive kit, but give that same awesome photographer a even better (read: more expensive) kit, and see how much their photographs will improve.