I agree with Lord Entrails- the absolute hardest initially was the distances, expressed in inches, that varied from indoor to outdoor.
Other things I remember-
1. The weapons table in PHB. You didn't have the internet to look those things up. Why can't a cleric use a lucern (sic) hammer?
2. Level. Character level. Spell level. Dungeon level (which also was, kinda sorta, CR). There were more levels than a construction site.
Weirdly, the negative AC thing always made more sense to me. Intuitively, for whatever reason, I always thought that the AC should go down.