So far I feel that the 3 most important things to understand really are:
-The Exposure Triangle of ISO, Aperture (FStop), and Shutterspeed, how they are related and their effects on each other.
-Understanding the effect of Aperture, Lens Focal Length, and Distance to subject on Depth of Field.
-Learning what your lens can and cannot do. IE Optimum Sharpness, Focal Length/Zoom amount, maximum and minimum aperture.
(The secret 4th thing is learning how to combat G.A.S. or Gear Acquisition Syndrome and stop spending copious amounts of money on things you don't need :P)
I've been putzing around for a little over 2 years now and all of that is just now starting to click and I find myself thinking more about correct exposure in Manual mode than spraying and praying in the Program modes.
Here's one I did the other day for a friend and his dad.