I would be willing to accept the proposition that we shouldn't have two sets of rules in this game because it would make it harder to learn.
But then how can Greg possibly justify the current system of Diminishing Returns? It is completely opaque. The categories don't make intuitive sense, only a very small numer of people even know those rules, and the default UI does everything it can to hide them from you in the first place. A lot of the problems with excessive CC in this game could resolved if this system were overhauled.
There ought to be significantly fewer categories, you ought to be VERY reluctant to allow one character to span multiple categories, and the UI needs to communicate to the player exactly what is happening.
One way to accomplish all of these goals in a simple, intuitive package is to have immunity timers instead. These can display as buffs on the player, and offer immunity from the form of CC suffered for a brief period of time. A typical example from other games might be that if you are stunned for 6 seconds, you are immune for 9 seconds (includes the 6). This creates more interesting game play because it means that there is actually a penalty for spamming CC in mass situations (like BG's) or for breaking it prematurely.
Regardless of whether Blizzard likes that particular solution - and it is certainly tested/proven in other games - I guess my main point is that you can't say, on the one hand, we want the game to be easy to learn, and then on the other hand have this awful/cumbersome/hidden DR system controlling the most frustrating aspect of the gameplay.