You earn Keys from battling regular duelists in the worlds, so which Keys you earn is completely randomized
Solemn Judgment is a last resort sort of card, but it's also extremely powerful because it can stop even the most powerful monsters from making their way onto the field
For example, a lot of the classic monsters in the original Yu-Gi-Oh anime series like Dark Magician, Blue-Eyes White Dragon, and Red-Eyes Black Dragon have archetypes based on them
Emerging onto Route 216 after using Defog in that random Mt Coronet pathway was a really special moment for me as a kid
Grimgrin frequently appears in live lots, where disciplined bidding can result in meaningful savings
Halvar can move Auras and Equipment around at will, and Sword of the Realms has incredible recursive value