For example, if an effect allows you to cast a sorcery spell with mana value 2 or less from among cards in your graveyard, you could cast Assault this way, but not Battery
1/4 Illustrated by Campbell White Standard Not Legal Alchemy Not Legal Pioneer Not Legal Historic Legal Modern Legal Brawl Legal Legacy Legal Comp
Almost entirely because of duplicate protection, they have now 100% lured me in as a customer to start seriously playing on MTG Arena and spending money, while also still collecting Common, Uncommon, Rare sets on MTGO (though no longer Mythics, it's too expensive to keep that up)
Several subreddits saw hundreds of comments where players were coming up with interesting combo potential and theorycrafting synergies, for everything from Hellkite Tyrant and Ichor Wellspring to Chandra, Spark Hunter
Others were printed like Grim Haruspex or the Reaper that cost less but only work for your stuff