For me, the biggest breakthrough was realizing why I had such a hard time with the Franklin-Covey system which basically says "In order to be truly effective, you really need to understand your values, roles, and general purpose in life before you start planning your day." I could never quite get a handle on those things -- too much ivory tower thinking.
GTD, on the other had, says "it doesn't really matter why you committed to it; the fact is, you committed to it, so let's figure out how to get it done."
After I got halfway decent at GTD (ok, maybe halfway is a bit generous, but I'm taking credit anyway), I realized that I could derive what my true values, roles, and mission were simply by looking at my current projects. By asking "why did I commit to this thing? what's in it for me anyway?" it was almost effortless to list those values and roles (the whole "mission" thing still eludes me: "be a good person... isn't that enough?")
This (relatively simple) effort has subsequently shaped the kinds of new things that I commit to, and as a result, I am committing to much less "junk" and I am a much happier person.
Thus completes the Circle of GTD.