The "a-ha" moment of functional programming.
I'd seen many people say that learning Lisp or Haskell would make you a better programmer, and that there was a brilliant moment where everything suddenly clicks.
At first I thought to myself "Bah, it's just rewriting loops as recursion. These people are probably just excited about finally understanding recursion."
But after a while I decided that I wanted to be sure. So I wrote a fractal program in Scheme. I thought to myself, "Well, that was interesting. But mainly it was rewriting loops as recursion."
I thought that was the "a-ha" moment. Clearly, I didn't get it yet.
This year, I went to a talk by Conrad Parker, who spent some of his talk on Haskell, and encouraged everyone to learn it. "Yeah," I thought, "OK." And I put some real effort into learning Haskell properly.
I think I had the real "a-ha" moment already, though maybe there's still a bigger "a-ha" moment on its way. Certainly I love Haskell and now I think the hype is justified.