Here is as many changes as I can think of. I'm going on D&D Rules Cyclopedia (BECM) vs. 2nd Edition because that's what I'm familiar with.
Ability Scores
* Modifiers go from -3 to +3 and are standardized across all scores. Bonuses start at 13. (unlike AD&D, where modifiers typically start at 15 and vary in increase speed). There is no exceptional Strength score, nor are non-warriors capped at a +2 Hp bonus for con.
* There are no secondary attributes for Bend Bars/Life Gates, Open Doors, Poison Save, System Shock, Ressurection Survival, Max/Min Spells Known, Learn Spell, Illusion Immunity, Max Spell Level, Bonus Spells, Spell Failure, Mental Immunities, or Thief Skills.
* Scores are typically aligned SIWDCCh not SDCIWCh
* PCs gain 5% bonus XP for a 13-15 in a prime score, and a 10% bonus to XP for a 16+.
Races
* There are only four races: Dwarf, Elf, Halfling, Human. Each race (with the exception of human) is treated as a class.
* Dwarves resemble fighters. They are limited to 12th level and have infravision and stonework detection.
* Halflings are also fighters, but have d6 hit dice and are limited to 8th level. They have bonuses to hit with missile weapons, AC vs. large creatures, and Init, as well as a sweet hiding ability.
* Elves resemble Fighter/Magic-Users. They are limited to 10th level (5th level magic) and have d6 HD. They can fight as well as clerics, wear any armor, and cast M-U spells in armor, but require double the XP of a normal fighter to advance. They also have secret door detection and infravision.
* Humans can chose any of the other classes, but beyond being a fighter, cleric, magic-user or thief, have no special characteristics.
Classes.
* All classes are built around the concept of 36 levels, not 20. Spell progression and Thief Progression are slower than AD&D, but eventually surpass.
* All classes used HD one die lower than their AD&D counterparts (except Magic-User, who can't go much lower than a d4). XP tables resemble AD&D tables, but are expanded to accommodate the 36 level cap.
* Clerics have d6 HD, use blunt weapons and all armor. They get thier first spell at 2nd level and no bonus spells for high wis. Turn Undead works on a 2d6 system (not d20).
* Fighters get d8 HD and nothing special except good combat ability. Specialization is discussed below.
* Thieves have a d4 HD. They can use any one-handed melee weapon and any missile weapon, but only leather armor. Thief Skills are pre-determined along a chart, and are miserablely low at low-levels. There is no bonus/penalty to skills for armor, dex or race. Backstab never raises above a x2 multiplier.
* Magic-Users have d4 and can only use daggers (though DM option can include staves, slings and whips). They automatically learn spells if they can afford to learn them. There is no school specialization.
* The RC introduced another new class: Mystic. Mystics resemble AD&D monks as unarmed fighters. They were always human but capped at 16th level.
* Fighters could, at 9th level and depending on alignment, become paladins (gaining some clerical powers), knights (gaining some nobility rights but no new class abilities) or avengers (kinda like anti-paladins). They could remain fighters if they chose.
* Clerics at 9th level, if neutral, could become druids. They gave up turn undead and the use of metal weapons/armor for access to druid spells (typically 4 new spells a level).
* There is no ranger, bard, barbarian, or illusionist classes, nor could you be a druid or paladin from first level.
Alignment
* Only three: Lawful (akin to LG/LN), Neutral (encompassing NG, CG, & N) or Chaotic (which was CE, NE and CN).
Skills
* Unless the optional system was in use, all classes were proficient in all weapons available to their class.
* The Optional System of Mastery added Weapon Proficiencies, but more importantly added levels of masters, from Unskilled (0) to Grand Mastery (5). Adding ranks required training and % chance of mastery. Mastery benefits varied by weapon but included extra attacks, bonuses to maneuvers like disarm and trip, AC bonuses, increased weapon dice, etc. Most Basic DMs found the system needlessly complex even compared to AD&Ds system.
* In addition, the RC introduced General Skills, which were akin to AD&Ds Non-Weapon Proficiencies.
Spells
* Clerics got 7 levels of spells, with 8 spells in each level. Some spells were at different levels (bless being 2nd level).
* Magic-Users (and elves) got 9 levels of spells (5 if elf) with 10-13 spells a level. Named Spells lost their name (aka Mordenkainen Sword became Sword).
* Despite the 36 levels, all caster-related effects capped at 20th level.
* Most spells were simpler than their AD&D counterparts (though high-level spells were just as complicated). Spells only had three factors: effect (which included size/shape), duration, and Save Type. There is no schools of magic, spheres, or Casting Time.
* Some spell effects changed slightly. Cure Light Wounds healed 1d6+1 damage OR paralysis. Magic Missiles were d6/missile.
Gear
* Most Mundane Gear, Gold Standards, and such remain unchanged (perhaps in cost).
* Weapons are simple, lacking speed factors and damage vs. large creatures. Weapons have a damage die, range, and weight. Many AD&D weapons are missing (flails & morningstars, for example) and some have different damage dice (bastard swords and warhammers, for example).
* Polearms come in three types: Polearm, Poleaxe, and Halbred. Stick that in your Glaive Gurasmie.
* There are 5 types of armor beyond clothing (AC 9), Leather (AC 7), Chainmail (AC 5), Scalemail (AC 4) Plate (AC2), and Suit (AC1). Most Basic dealt with Chain, Leather and Plate. Shields lowered AC by 1. AC began at 9, not 10.
Combat
* Surprise and Initiative are both d6s. Few things modify it. DMs have the option of group or individual Init.
* Fighters at high level (as well as elves, dwarves, monks and halflings), gained special Fighter Options which included Trip, Disarm, Parry (deflect an attack) Smash (penalty to attack for adding your strength SCORE to damage) and lance attack. Multiple attacks were possible against foes with very low ACs vs. your to hit bonus.
* Saves were as AD&D, except some names were simplified (Turn to Stone vs. Petrify) and Wands used the same column as Spells. Wisdom Modified all saves vs magic.
* To Hit was done via a table system. Demi-humans who capped at 8-12th level could gain attack "ranks" which increased their to hit, but not other level-dependent features.
* There was very few situational bonuses. A few for flank, retreating, and prone attacks were it.
* Two-Weapon Fighting and Unarmed/Subdual Attacks were OPTIONAL systems.
Monsters
* Most common AD&D monsters are there, but most were simplified. Few had multiple attacks, spell-like abilities, and such.
* Most poisonous creature were fatal.
* There are no demons/devils, but creatures from "nightmare realities" could fill the niche.
* Giants are beefed up is in 2nd edition.
* Liches could be clerics.
* Dragon come in Blue, Black, Green, White, Red and Gold. Not all dragons are evil. They come in Young, Adult and Old ages, and don't cast clerical spells.
* XP was fixed based on the creature.
* There is no % Spell Resistance, except in unique monster situations (like raksashas).
* Morale was set on 2d6.
* Pseudo Creature Types were included (Undead, Humanoid, low-life)
Magic Items
* Most magic Items had one function, not multiple. IE there was a wand of fireballs, but not a wand of fire that created multiple fire-based spells.
* Rings and Cloaks were the only thing that increased M-U AC. No Bracers!
* Generally Speaking, there were few "special" weapons in the books like Holy Avengers or Luckblades. No weapon/armor scaled over a +3.
* Scrolls could do a lot more than hold spells.
* Magic Item Creation Rules and Spell Research Rules Included.
Other Stuff
* Basic lncludeded some of the BEST Dominion/Stronghold Rules, Mercenary/Henchmen Rules, and Mass Combat Rules. Complex? Yup. Better than Anything AD&D came up with? Youbetcha!
* Gazetteers and other Supplements added new monsters, spells, and classes.
* The RC included some sketch rules for converting to AD&D 2e and back.
* Basic Introduced us to a variety of monsters that only later appeared in AD&D/3e/4e: Aranea, Nightwalkers, Blackballs/Umbral Blots, Rakasta, and others.
I probably missed a few things, but that's a through as I can think of!
