I'm surprised that no mention was made of the big rewrite. A lot of us early users were lisp/Graham followers and intrigued by the fact that the site was written in lisp. I seem to recall the rewrite being a big deal.
We did a post-acquisition python-to-python rewrite -- clean out the cobwebs, switch frameworks, and redo the database schema -- which was much harder, longer, and more painful to deploy. Though in that case it was one of those drastic changes whose primary side effect was that no none notices a thing.