My number one priority: Players feel that their losses were fair. Not the fault of an unforeseen card.

Every color gets to range from aggro to control, to some degree. Blue and Red are not forced to play just one way.

Every card in the list has distinct tradeoffs with every other, and there's a believable path to any card being picked over anything else. So Llanowar Elves prevents Arbor Elf from coming in for being too similar. Lightning Bolt is too much better than its competition, so it gets broken up into Volcanic Smite and Torch the Tower.

Removal has been downtuned to prevent it from invalidating interesting high drops. Swords to Plowshares and Cut Down are replaced by Thraben Charm and Wither and Bloom.

Gold cards are or create permanents so you can feel yourself accumulate value for that extra commitment.

There should be little to no repetitive gameplay loops. So no Crystal Shard.

Graveyard value, blink, and +1/+1 counters are not really supported but have pieces to be substrategies (e.g. Lively Dirge, Oji, the Exquisite Blade, and Metastatic Evangel).

Trinkets such as Powerstones and Blood have been getting an absurd amount of support lately and I have a strong appreciation for what they offer to the game. But I've taken special care to exclude cards that just make it a bland tribal counting exercise (Inventor's Apprentice) and instead use artifacts as a resource (Reckless Detective) or let creative cards flourish because of them (Builder's Talent).