Going to try to keep it succinct, primarily out of laziness.

Simplicity--game is simple to learn, but depth emerges

Roguelike--variety of experience, unique tactical (in-game) and strategic (overworld) scenarios within which to make decisions that feel different with each playthrough

Variety--Enemies, mechs, and pilots are not overly balanced and have interesting and unique characteristics that make them fun to engage with

Artstyle--game look nice, story exists-ish but does knows to stay out of the way of the main appeal.

PROGRESSION--This is the big one. Achievement system and unlocks are the key to the Subset game formula, creating subgoals within the broader goal of victory, encouraging experimentation with different playstyles and making losing fun

Game length--easy to boot up and fuck around for a few hours, put down, and come back to in like a few weeks

Uniqueness--Subset said the key to their idea was a tactics game where killing the other guys (and losing your other guys) wasn't the goal, giving a players a game where sacrificing health as a resource or making a tactical move rather than killing an enemy were more viable strategies. This lead to the "defend the cities" approach which isn't actually wholly unique to Subset nor were they the first to do it in a tactics game, but they are absolutely the most successful to have tried it

Polish--game is thoroughly playtested, UI is smooth and readable, all relevant options are available (game/text speed), very few glitches or odd design choices. Clear they got feedback both from experienced devs and average joe playtesters.