There were a lot of factors: figuring out what people wanted out of a D&D set, how to fit that into competitive play environments, and how to capture the essence of everything on the cards
Samwise Gamgee It seems wrong that perhaps the most likely degenerate card in this set is Samwise Gamgee

A Girl and Her Dogs (playtest) A Killer Among Us A Most Helpful Weaver Aang, Airbending Master Aatchik, Emerald Radian Abdel Adrian, Gorion's Ward Abian, Luvion Usurper (playtest) Abstruse Interference Abundant Countryside Abyssal Harvester Abzan Ascendancy Abzan Monument Academy Manufactor Accomplished Automaton Acererak the Archlich Aclazotz, Deepest Betrayal Acorn Catapult Adagia, Windswept Bastion Adanto, the First Fort Additive Evolution Adeline, Resplendent Cathar Adeline, Resplendent Cathar Adipose Offspring Adorned Crocodile Adorned Pouncer Advent of the Wurm Adverse Conditions Aerie Worshippers Aether Channeler Aether Chaser Aether Herder Aether Inspector Aether Poisoner Aether Refinery Aether Swooper Aeve, Progenitor Ooze Afterlife Afterlife Insurance Agate Instigator Agatha's Champion Age of Ultron Agent 13, Sharon Carter Agents of HYDRA Aggressive Biomancy Ainok Strike Leader Air Nomad Legacy Ajani Fells the Godsire Ajani Goldmane Ajani's Chosen Ajani's Last Stand Ajani, Nacatl Avenger Ajani, Nacatl Pariah Ajani, Outland Chaperone Ajani, Strength of the Pride Akim, the Soaring Wind Akki Ember-Keeper Akoum Stonewaker Akroan Crusader Akroan Horse Alandra, Sky Dreamer Alela, Artful Provocateur Alela, Cunning Conqueror Alenni, Brood Recruiter Alharu, Solemn Ritualist Alicia Masters, Skilled Sculptor Alien Invasion Aligned Heart Alirios, Enraptured Alistair, the Brigadier Alive // Well Alive // Well Allied Teamwork

Ill-Timed Explosion We are stupidly glutted for board wipes in Standard and theres not much Izzet action, but Ill-Timed Explosion is super powerful
What affects the value of The Aetherspark
His inspiration for making it a proofreader came from the white activation that deleted letters from the names of permanents (the original version just hit permanents, but Ari later came up with the idea of adding players, as that seemed like it could lead to some fun interactions)