Stripping down AD&D (2nd Edition) to the bare essentials

To illustrate how much tinkering AD&D 2nd Edition can handle without becoming unbalanced or incomplete (in the way that 3.5e seems to become), I'm just going to make a list of all the optional and tournament rules in the Player's Handbook and Dungeon Master Guide, any of which can be left out completely and still leave the game fully playable. The ones in italics are explained only in the DMG.

Chapter 1: Ability Scores

• Maximum Number of Spells per Level (Optional)

Chapter 2: PC Races

• (technically, Multi-Class and Dual-Class aren't marked as "optional", but a DM certainly doesn't have to allow them)
• Slow Advancement (Optional)
• Exceeding Level Limits (Optional) 

Chapter 3: PC Classes

• Creating New Character Classes (Optional)

Chapter 5: Proficiencies (Optional)

• (the entire chapter is optional, except for the rules on tracking which a ranger needs) 

Chapter 6: Money and Equipment

• Encumbrance (Optional) (divided into the following subcategories)
• Basic Encumbrance (Tournament)
• Specific Encumbrance (Optional)
• Encumbrance and Mounts (Tournament)
• Horse Traits (Optional)
• Armor Made of Unusual Metals (Optional) 

 Chapter 7: Magic

• Spell Components (Optional) (it can be assumed that every spell is V+S+M, like in B/X)
• Determining Spell Components (Optional)
• Researching Extra Wizard Spells (Optional)

Chapter 8: Experience

• Individual Experience Awards (Optional)
• Training (Optional)  

Chapter 9: Combat

• Weapon Type vs. Armor Modifiers (Optional)
• Group Initiative (Optional) (this refers to modifiers, which are all technically optional)
• Individual Initiative (Optional)
• Weapon Speed and Initiative (Optional)
• Pole Arms and Weapon Frontage (Optional)
• Shields and Weapon Frontage (Optional)
• Critical Hits (Optional)
• Parrying (Optional)
• Specific Injuries (Optional)
• Hovering on Death's Door (Optional)
• Aerial Combat (Tournament)
• Aerial Combat (Optional) 

Chapter 10: Treasure and Magical Items

• Artifacts and Relics (Optional)

Chapter 14: Time and Movement

• Jogging and Running (Optional)
• Terrain Effects on Movement (Optional)

Appendix 3: Magical Item Descriptions

• Command Words (Optional)

So, in total, I only see myself using a few of these rules at first: Basic Encumbrance, Individual Experience Awards, Hovering on Death's Door, Jogging and Running, and Command Words, along with the set of Secondary Skills from Chapter 5. I definitely will employ Exceeding Level Limits if my players ever reach the higher levels with demihuman characters.