So I stumbled across this article today: 

http://www.hooktheory.com/blog/i-analyzed-the-chords-of-1300-popular-songs-for-patterns-this-is-what-i-found/

This person ran an analysis of 1300 popular songs and found surprisingly little variation in their chord progressions.  On average, 75% of the chords in a typical pop song are the I, IV, and V chords.  Add in the minor ii, iii, and vi chords, and you’re over 90%.  And that vii diminished chord that I always say you won’t run into very often?  Doesn’t even make a statistically significant appearance.  

Actually I don’t think there’s anything particularly surprising about this.  If you were to do a similar analysis of jazz songs, you’d find a MUCH wider diversity of keys and chords.  But don’t interpret this as a negative thing–there is nothing wrong with simplicity.  In fact there’s a lot of beauty to be found in simplicity. 

The big takeaway here, in my opinion, is that the basic level of music theory (what is covered in the HCG Music Theory Primer) is sufficient to understand 90% or more of what you run into in popular music.  In other words, theory does not have to be rocket science.  Just get a basic grip on chords and chord progressions, and there will be very little that you will not understand about pop music.