Holmes Basic: Both the cover and sample dungeon in tbe book. If I recall correctly, there was also the abiility to lower certain abiity scores to raise others.
B/X: The lower ability score modifiers
AD&D 1e:
B/X: The lower ability score modifiers
AD&D 1e:
- Several of the modules (C1, D1, D2, G1-G3, I6, N1, S2, T1),
- Greyhawk as presented by Gygax, the PHB cover.
- The feel of D&D if one ignored things like S3: Expedition to the Barrier Peaks and several of the
- Priests of Specific Mythoi/Specialty Priests
- Clerical Spheres,
- Complete FIghter's Handbook, Complete Thief's Handbook, Complete Priest's Handbook, Complete Druid's Handbook
- DMGR series
- HR series,
- Dark Sun (original box set), Land of Fate box set, Ravenloft: Realms of Terror boxe set
- PO: Combat & Tactics, PO: Spells & Magic
- Unified Resolution Mechanics
- Ascending AC
- Unified Leveling Progression (although I think spellcasters needed to be tuned down)
- Three save categories,
- skill points (although no class should have less than 4+Int Mod per level unless the class is based on Int)
- class variants despite them getting little love until Unearthed Arcana
- 0/0 level multiclassing at first level (in 3.0 DMG)
- variants in the DMG
- critical hits
- Mass Damage Save
- monsters get ability scores
- monster AC breakdown (although the number of modifier types got out of hand)
- Unearthed Arcana, Fiendish Codex 1
- third party support
- the removal of some non-biological abiilities from race and moving them to feats
- Ranger as non-spellcaster
- martial characters getting interesting things to do,
- starting class builds (e.g. Artful Dodger, Brutal Scoundrel Rogue, Cunning Sneak)
- multi-class feats, magic missile requiring attack roll, the power system (in theory)
- Backgrounds
- Advantage/Disadvantage
- Inspiration
- Variant Human
- Bard class (finally closer to my image of a Bard)
- Battlemaster Fighter
- some of the third party support and fan material