You bring up cryonics and AI. 25 years ago Engines of Creation had a chapter on each, plus another on... a global hypertext publishing network like the Web. The latter seemed less absurd back then than the first two, but it was still pretty far out there:

One of the things I did was travel around the country trying to evangelize the idea of hypertext. People loved it, but nobody got it. Nobody. We provided lots of explanation. We had pictures. We had scenarios, little stories that told what it would be like. People would ask astonishing questions, like “who’s going to pay to make all those links?” or “why would anyone want to put documents online?” Alas, many things really must be experienced to be understood.

I believed Drexler's prediction that this technology would be developed by the mid-90s but I didn't expect it to be taking over the world by then. Probably to most people even in computers it was science fiction.

As far as computers in general, their hardware reliability's the least intuitive aspect to me. Billions of operations per second, OK, but all in sequence, each depending on the last, without a single error? While I know how that's possible, it's still kind of shocking.