Four decks, each built on a returning mechanic: morph, madness, flashback and populate.
Cast a creature face down as a 2/2 for three, then turn it up later for its morph cost. The Sultai deck lets you do the first one each turn for free, which turns a clunky mechanic into a real tempo plan.
If you discard a card with madness you may cast it for its madness cost instead of putting it in the graveyard. The Rakdos deck loots repeatedly, so discarding stops being a cost.
Cast a spell from your graveyard for its flashback cost, then exile it. The Jeskai deck treats the graveyard as a second hand and copies the first spell it casts each turn.
Populate copies the best creature token you control. It scales with token quality rather than quantity, which is why Ghired making a 4/4 Rhino matters so much.
Commander 2019 is the batch where each deck was built around one returning mechanic rather than a color identity or a plane. That gives the four boxes unusually distinct textures for a single release.
It is also the release that produced two of the more durable precon commanders. Anje Falkenrath turned madness into a genuine engine, and Kadena made morph playable by removing the cost of the first face down creature each turn. Both are still played, which is not something you can say about most 2019 precon cards.
The set has aged well in one other way: none of the four plans depend on cards printed since. Morph, madness, flashback and populate all have deep back catalogs, so upgrades cost very little.
Merciless Rage is the strongest engine in the batch and the cheapest to improve. Faceless Menace is the most fun to play against people who have never had to guess what a face down 2/2 is.
Kadena, Slinking Sorcerer
A Sultai morph and face-down creature deck led by Kadena, Slinking Sorcerer, cast face-down creatures for free the first time each turn, draw a card on each face-down entry, and keep opponents guessing until you flip your worst nightmares.
Anje Falkenrath
A Rakdos madness deck led by Anje Falkenrath, rummage endlessly with haste, discard madness cards for their cheaper cost, and untap Anje for every madness card to chain through your entire hand in a single turn.
Sevinne, the Chronoclasm
A Jeskai flashback and spells-from-graveyard deck led by Sevinne, the Chronoclasm, copy the first instant or sorcery you cast from the graveyard each turn for free, and build a value engine that makes every flashback spell twice as powerful.
Ghired, Conclave Exile
A Naya populate tokens deck led by Ghired, Conclave Exile, create a 4/4 Rhino token on entry, populate it on each attack to create another copy, and snowball a board of large trampling tokens that opponents can't keep pace with.