This old poll popped into my mind tonight, and I thought I'd write a scorecard for what made it in. Quite a lot, actually!
(YES) Intelligent and Ego weapons 62.65%
(YES) Strongholds 60.24%
(YES) Cursed items that look like good magic items 56.02%
(YES) Morale for monsters 55.42%
(YES) Morale and loyalty for henchmen/hirelings 47.59%
(YES) Wand of wonder 45.78%
(NO) Disease track 44.58%
(YES) Secret languages 41.57%
(YES) Reincarnation 40.96%
(YES--Sorcerer) Wild mages 37.35%
(YES) Potion miscibility 36.75%
(YES) Assassin 34.94%
(YES) Tieflings (Planescape) 33.13%
(YES) Insanity 33.13%
(YES) A "scaly" race (e.g. dragonborn or lizardfolk) 33.13%
(YES) Modrons 33.13%
(NO) Name level reward 31.33%
(NO) Prostitute encounter subtable 30.12%
(YES-feat) Heavy armor grants a small amount of DR 28.92%
(NO) A large race (e.g. goliath or AU giant) 27.71%
(NO) Brutish dragons 27.71%
(YES) Cambion 27.11%
(YES) Forced movement in combat 27.11%
(YES--Champion fighter) Swordsage class 26.51%
(NO) Anti-magical dwarves 25.30%
(YES) Silly monsters 25.30%
(YES--mephits) Para- and quasi-elementals 25.30%
(NO) Racial levels (e.g. paragon, Arcana Unearthed) 22.89%
(NO) System shock and ress. survival 22.89%
(NO) Allegiances 22.29%
(YES) Cube of Force 22.29%
(NO) Certain weapons have bonuses vs armor types 21.69%
(YES-optional) Initiative modified by weapon speed and casting times 21.69%
(NO) Jabberwock 21.08%
(NO) Druidish bards 21.08%
(NO) A "furry" race (e.g. rakasta or sibbecai) 19.88%
(NO) Ki-Rin 19.88%
(NO) A tiny race (e.g. AU sprite) 19.28%
(NO) Ability score prereqs for race/class 18.67%
(NO) Binder class 18.67%
(NO) Dueling combat subsystem (spell or melee) 17.47%
(NO) Lynnorms 14.46%
(NO) Xvart 13.25%
(NO) Akashic/Factotem (the super-Expert) 12.65%
(NO) Incarnum classes 9.64%
(NO) Thoul 9.64%
(NO) Xaositects 7.23%
(NO) Duel an NPC to level up 7.23%
(NO) Illumians 4.82%
(NO) Red steel 4.82%
(NO) Algoid 4.22%
(NO) Diaboli (Mystara) 1.81%