Here are couple of points I never really knew, or knew but not as specific elements of the rule books. Starting with the dungeon masters' guide, I didn't know how much Gary Gygax wanted to kill off my character. Aside for elements like monsters and wizards having no hit points, Gygax was a butcher. Those Disease and Parasitic infection rules were basically setting a timer on characters lives. You apparently adventured solely in the northern climates or awaited renewal in carousel. Seriously Just hanging out in a tropical city allowed for the chance of infection every week. Let alone going through sewers like you might in Gygax's A series. Yes I survived and conquered but I croaked due to scabies.
Another point I had ignored was how much TSR was trying to sell to me. They sure mention all their little figures and how useful they would be. Dungeon Geomorphs shit you better have em, son! I sort of remember TSR going a bit apeshit about their license back in the late 80's with the whole Role-aids thing. I had no idea they had started it with The Judges Guild license. Accept no substitutions! I can understand that in some respect, quality control is handy. Our Dungeon Master Brian had all sorts of extra stuff hidden in binders that took the original game and blew it out of the water. But flipping out over a module, whatever.
Back to the players handbook, thieves should never exist anywhere. A first level thief with the best chance to pick pockets is only going to have a 35-40% chance of ripping you off. How do they get better starting off that low. I mean you do it once and retire I guess.
There are a few rules I seem to have materialized from out of my head. I would swear I have read that you can only receive so much experience from an adventure. That number was specifically enough to level you and then you could reach the next level minus a single experience point. I can tell you that rule has been followed from gold box games to Dungeons and Dragons online. But I have no idea where it is in the Dungeon Masters guide.