(Caution: not for the softcore)
Alignment: AD&D employed a notorious form of psychometric profiling, the Alignment System, whereby every living thing’s personality could be summarised by a phrase like “Lawful Neutral” or “Chaotic Good”. The two axes of alignment dealt with reaction to authority and moral outlook: these were Gary Gygax’s only concession to the idea that characters should have a personality. Absurdly, each of the nine alignments had its own language which only similar people could understand – various glosses tried to explain this idea, settling on the notion that “alignment language” was sort of like body language. Bad people, in other words, smile less.
Hit Points: HP are now such an accepted part of computer gaming that it’s odd to remember how disliked the system was in the 80s roleplaying era. The problem in AD&D was that while a character’s hit points rose with experience, the capacity of weapons to do damage stayed pretty constant. The implications were that a higer-level mage wearing only a robe, for instance, could survive several maximum-damage blows with a broadsword which would immediately kill a lower-level character wearing full armour. This made it very difficult for the referee to actually describe what was happening in combat. “You are hit with a crossbow bolt. No ill effects though.” Again, exegeses of these rules were common: one idea was that only the last 10 or so ‘hit points’ were real, the rest represented a character’s luck gradually running out in a fight. (There was also another rule whereby your HP could go down to -10 before you actually died, or something. Nobody used it.)
Economics: The unit of currency in AD&D was the gold piece. There were also silver pieces, worth 1/10 of a gold; copper pieces worth 1/10 of a silver, and platinum pieces. Worth 10 times a gold piece, you’d think? No. Five times. And then there was the electrum piece, worth half a gold piece. All these different coins were accepted without question anywhere in an AD&D world – who minted the coins, and how this oddly complex five-tiered system had evolved, and why the electrum piece even existed other than to show that Gary Gygax knew the word “electrum”… these questions were left to the individual referee’s discretion.
Greed Is Good: You could get experience points in two ways in AD&D. If you killed a monster, you got some XP. And you got 1 XP for every gold piece you found. I am pretty sure you had to find them – you couldn’t just bank them and grow powerful from the interest. But on the other hand it was also implied you had to actually get the things home, just seeing and planting a metaphorical flag on the treasure wasn’t enough. Why did having gold improve your fighting or magic skills? Don’t ask that. The general thinking was that the experience-for-gold trade-off represented a boost from completing a particular mission, and most adventures were designed to follow this logic, with the big pay-off at the end. The obvious solution – of just giving XP based on how well an adventure went – was forbidden by Gygax, though nearly every ref just ignored him.
Encumbrance: The gold-as-XP logic of the AD&D gameworld meant that characters would routinely be faced with the problem of getting several tons of cash from place to place. The rulebooks were quiet on the logistics of this, but did include an extensive ‘encumbrance system’ to simulate the effects of excess weight on movement and combat. Of all tables in the AD&D rules the Encumbrance Tables were the most widely ignored* – referees quickly worked out that even trying to take them seriously would render characters completely immobile.
*actually, no, the Armor Class Modifier Tables were the most ignored, but of them we need not speak.