I was very pleasantly surprised to see the AMA announcement on Reddit's frontpage, given how relatively non-mainstream the S.I. is and how many page views Reddit gets (and gives).
Also, although there is a large inferential distance between Luke and most Redditors (as siodine noted), I thought Luke did a great job trying to bridge the intuition gap--with the usual abundance of links and all.
I'm sure most of us are used to just being able to badger him about things in the comments here on LW, but for anyone interested here's the link.