One big thing is you never even knew what you didn't know. I had a C=64 for years and never even heard of the PEEK and POKE commands until years later when I was reading about it on the web. There were no computer experts to talk to back then, at least not where I lived. If it wasn't in the book or discoverable from just fiddling you weren't likely to find it.

The worst part for me is that my library had a computer section, but it was filled with stuff like "FORTRAN for System/360". Apparently the library decided that they had enough computer books and didn't bother getting new ones.