Solemn Simulacrum just feels good to cast, and feels like its on the correct side of the mana curve for the value it offers as a colorless card
It also allows you to cast all of your green creature spells as though they had Flash
While its plenty powerful, its the effect on perceived barrier-to-entry that really posed a problem because casual players watching Commander games in passing could reasonably assume that they needed hundreds (now thousands) of dollars in Power-9 mana as table stakes, just to join the format
Due to their often higher mana costs and splashy effects, many of the more modern Hydras have bypassed formats like Standard, Pioneer, and Modern, and were printed directly into Commander sets
Im even going to try this one out: Creatures (23) 4 Rattleclaw Mystic 3 Whisperwood Elemental 3 Den Protector 2 Deathmist Raptor 3 Nissa, Vastwood Seer 4 Hangarback Walker 2 Oblivion Sower 2 Ulamog, the Ceaseless Hunger Planeswalkers (3) Lands (26) Spells (8) From Beyond gets better when the 1/1s actually threaten to trade with stuff and when you go up on fetchable Eldrazi options
Some case file showcases also come with invisible ink scrawled in their text box, though the invisible ink versions are Collector Booster exclusives