Well, this might get me voted off of the island, but I have to say that the most noticeable and beneficial dietary modification that I have ever made was when I eliminated caffeine from my diet earlier this year. I had cut down a few times before, but never gone caf-free. After the first ugly stage of withdrawl, I was shocked by how much energy that I seemed to have all of the time. I also found that I wasn't as sleepy after a short night of sleep. I imagine that my adrenals were pretty stressed, and my expresso habit, which worsened while I was dating my local barista - certainly didn't help them.
Now, I still love the flavor, so I still drink my decaf. A good blend is really important here, though. I also drink herbal coffee sometimes.