I've been playing 'D&D 'ever since I was a young boy and found my dad's Dungeon Master's Guide on his bookshelf (the original, by Gygax). I was still too young to understand a lot of it (Gygax opens with a discussion of uniform vs binomial probability distributions...), but I spent hours of days flipping through the pages, looking at the pictures and reading the descriptions of a fantastic world. It inspired me to make my own games about exploring through planned dungeons with friends and having them use ability scores to get through.
By the time I was old enough to try to really figure out the rules, version 3.0 was out, which made a lot of changes. One of the biggest was replacing the original Descending AC and THAC0 system with the slightly more intuitive Ascending AC system. This system always just made sense to me, whereas DAC and THAC0 always seemed weird and complicated. I never understood how DAC worked, and never really saw a good explanation of it, and so just never used it. It was only recently, when getting into old school tabletop RPGs, that I decided to look up an explanation of how DAC and THAC0 used to work.