Sadly, I don't have a list of the Sacred Cows that were identified by Jonathan Tweet and the 3rd edition design team. Therefore, I'll create my own take on what must be retained in D&D Next.
At least one of the AD&D 2nd edition sacred cows was partially slaughtered, or at least turned upside down. We still use 20-sided dice to resolve combat attack rolls to determine whether they hit or miss. THAC0 (or the "To Hit Armor Class 0" target value) has been removed from the D&D game but the change was so seamlessly brilliant that it's only the calculation that's different. There are some slight differences that may lead one or another to prefer THAC0 as a system but those differences aren't material.
Sacred Cows in D&D Game Mechanics:
- Fantasy Races
- Hit Points
- Armor Class
- 1d20 Combat Roll
- xdx Damage Dice
- Class Levels
- Monsters (and NPCs)
There remains a lot of detail within those Sacred Cows that is flexible. In order to have a D&D flavor, the rulebooks would need to have a minimum of four classes: Fighter, Thief, Magic-user, and Cleric. The more I think about it the more I think that extra classes, beyond those four, are good for the game but I'd say those are probably the minimum. The attributes (currently Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma) have fluctuated over the history of D&D. I do not think the current list is needed exactly but I think something roughly similar would be needed to meet this Sacred Cow requirement. I have not included magic items on this list because I have played in many low-magic D&D games that featured little to no such items but I think weapons and armor are core to the experience.
One last note, what other explorations of D&D are interesting to you, the readers?