It's been a long time since it was a serious issue for any format besides Pauper, but take a time walk, and you'll see Affinity is one of the most busted Magic: the Gathering mechanics in the game's history. Reducing the cost of spells for each artifact you controlled was just too powerful in a set full of cheap artifacts and artifact lands.
But now an article from Wizards about the design for the Mirrodin MTG set, where this dangerous keyword made its debut, shows things could have been so, so much worse.
The Design Files: Mirrodin piece posted by the set's design lead Mark Rosewater on March 2, looks at Mirrodin's design handoff file, revealing early card designs as they stood before the development team got to fiddle with the set for about half a year before release.
And as it turns out, affinity was originally going to be far more commonplace. Mirrodin only used the mechanic on blue or colorless cards, but Rosewater reveals it was originally found in every color except green, and appeared on way more than the eight affinity Mirrodin cards featured in the final set.
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Wizards' article only shows the white and blue prototype cards from Mirrodin, with the other colors to follow in a 'part 2' piece. But this already reveals plenty of scrapped affinity cards which could've made the artifact-heavy strategy even more broken.
For instants and sorceries with affinity, we have a white exile effect and a blue bounce spell. For creatures, white got a 10-mana flying angel 5/5 with affinity for artifacts and for plains, and also protection from artifacts.
White didn't get to have all the fun though, as in blue there was a six mana affinity wizard which tapped to counter a spell, unless its controller paid one mana for each artifact you controlled. Twisted!
It gets worse though. There's an eight mana draw-three spell which, as Rosewater points out, would function as Ancestral Recall if you had enough artifacts on the field.
Scariest of all is a two mana blue sorcery, which in the design file is named Artificial Enhancement. This card gave all your artifacts affinity for the rest of the turn. Presumably, decks would've played this card and then immediately unloaded an entire hand's worth of artifact creatures, making the spell function like a two-mana Omniscience.
Rosewater's comment is something of an understatement: "Knowing how affinity for artifacts played out after Mirrodin's release, I'm glad we didn't print this card."
Affinity for artifacts created an unpleasant Standard environment sometimes described as the 'Second Combo Winter'. While things didn't get truly nasty until the second Mirrodin block set Darksteel which added the card Arcbound Ravager, Mirrodin set the stage for a broken archetype, thanks to its artifact land cards which made affinity far too easy to exploit.
Ultimately, the mechanic led to a mass wave of card bans in 2005, with eight cards stuck on the MTG banlist at once (including the entire Mirrodin land cycle).
Along with this revelation about Affinity, the Mirrodin design file also contains numerous cards using Energy. That's surprising because this mechanic didn't actually show up in Magic until Kaladesh, meaning it was thought up in 2003 and then shelved for 13 years!
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