It isn't any specific cards that have really changed things, Fatal Push would be the biggest contributor if I had to name one
(You may tap a Vampire you control in addition to any other costs as you cast this spell.) Target opponent loses half their life, rounded up
It spreads rad counters to all players and once they start milling cards due to the radiation, you make some 2/2 mutant zombies
The "2/0" in the top of the corner means that it costs two to cast and zero to activate
That said, it commits you to going heavy red, the worst color, and at any point in the game past the first few turns its quite weak
These modes are less daunting than drafting, as they allow players to bring in their decks rather than create them as part of the event