overgeeked
B/X Known World
As much as I can, honestly.
The already mentioned skill challenges (with tweaking), monster roles & templates, minions, bloodied condition, along with a one-page chart for monster stats, rituals (anyone can cast non-combat spells with a scroll, ritual book, paying cost, etc), PC power sources & roles (primal defender, arcane leader, etc; mostly used as descriptors and treated like Fate Aspects to give narrative permission and dis/advantage on relevant rolls), the World Axis cosmology, the points-of-light concept (almost always), the Nentir Vale setting (frequently), all the advice in the DMG1 and DMG2 (they're honestly the best DMGs ever published for any edition of D&D), residuum, etc.
The already mentioned skill challenges (with tweaking), monster roles & templates, minions, bloodied condition, along with a one-page chart for monster stats, rituals (anyone can cast non-combat spells with a scroll, ritual book, paying cost, etc), PC power sources & roles (primal defender, arcane leader, etc; mostly used as descriptors and treated like Fate Aspects to give narrative permission and dis/advantage on relevant rolls), the World Axis cosmology, the points-of-light concept (almost always), the Nentir Vale setting (frequently), all the advice in the DMG1 and DMG2 (they're honestly the best DMGs ever published for any edition of D&D), residuum, etc.