"Pessimism" or "optimism" is a wrong way of describing the disagreement, there are important details missing in such broad a categorization, and if we don't agree on methodology of making the estimates, comparing the resulting numbers is useless. For example, How Likely is Cryonics to Work employed a mistaken independence assumption that makes the quantitative conclusion of the analysis meaningless (even as some of the intermediate steps are more informative).
"How Likely is Cryonics to Work" employed a mistaken independence assumption
It didn't:
All my probability guesses are conditional on everything above them not happening. For example, society collapsing and my cryonics organization going out of business are very much not independent. So the probability assigned to the latter is the chance that society won't collapse, but my organization goes out of business anyway.
There was some discussion of this on the post.
When I've brought up cryonics on LessWrong [1][2], most commenters have said I'm being too pessimistic. When I brought it up yesterday at the Cambridge MA meetup, most people thought I was too optimistic. (I think it could work, but there are enough things that could go wrong that it's ~1000:1 against.) What makes the groups so different on this?
[1] Brain Preservation
[2] How Likely is Cryonics to Work