I'm curious as to what would have been your original probability estimate for "Given that Eliezer writes a Harry Potter fanfiction, it becomes the most popular Harry Potter fanfiction on the Internet."

I would have assumed that the proper reference class was all your other fiction, which as much as I enjoyed them, were all short compared to MoR even the popular ones like 'Three Worlds Collide'. (One of my favorites, the Haruhi fanfic, was, what, 2 pages on FF.net?) Short fanfics cannot become the most popular, so I would have assigned it a very low probability. Had you asked an estimate for 'wrote a full trilogy of HP fanfic novels', my estimate would be quite different. I won't pretend to know what it would have been. (I remain surprised and amused that it has become as large and popular as it has been, and that you now procrastinate on both your rationality book(s) and the Center by writing MoR. Truly, fortune passes everywhere!)

Or, for that matter, getting out of the AI Box.

Oh, I would've bet for you back on SL4. You already had succeeded in getting SIAI running, after all. (Although here again the counterfactual gets hard - I barely remember what I was like back in 2003-4 when I joined SL4 and IIRC you had already won an AI Box by then.)

Offer 20-to-1 odds against HPMOR winning Best Novel and I'll buy in.

Sure. I am poorer than a church mouse, so I can risk no more than ~$100. How's this:

"For any Worldcon 2013-2017, MoR will win the Hugo 'Best Novel' award. The stakes will be $100 against $5 (non-inflation adjusted). Payment by either Paypal or Bitcoin (at that day's exchange rate on the largest exchange eg. Mt.gox) to the winner or a charity of the winner's choice. In case of any dispute, the verdict will be judged by Carl Shulman or another person mutually acceptable to Eliezer Yudkowsky and gwern."

Hm, now I'm curious as to which side of the bet Zvi or Kevin would take.

I've pinged them.