Magic: The Gathering

The biggest problem with Arena is no Rule 0. You can go in with your fun Madness deck you theory crafted, and go against an Eldrazi or Tiamat and have your day ruined. Then when you create your soulcrusher stax deck, you'll get put against someone playing a relatively benign mill deck.

Then that person decides to make their own soulcrusher deck, and more and more people create decks that make people never want to play on Arena again.

If it were possible to look at an opponent's commander before entering the game, that would solve at least some issues, i think. I'd personally never play against an Ulamog, Ugin eye of Storms, or Tiamat