It started as blocks of tofu
The prototype had no squids in it at all. Two teams of plain white rectangular blocks — the developers called them tofu — squirted INK around an arena, and the game was already enjoyable before anyone had decided what the characters were.
The SQUID arrived later, and solved several problems at once. The team needed a reason a character could swim through PAINT and disappear into it, and an animal that squirts ink and squeezes through tiny gaps answered every question. The INKLING, half kid and half squid, was designed backwards from a mechanic.
You can win without hitting anyone
A TURF war is scored on floor, not on knockouts. After three minutes the game measures how much ground is covered in each TEAM's COLOR, and that is the result. Splatting an opponent is useful because it buys space and time, not because it scores a point.
That changes who can play. A cautious player quietly painting along the edges contributes as much as the one charging into every fight, and a beginner is never dead weight. NINTENDO set out to make a shooter suitable for a nine-year-old, so nobody bleeds, the splatted are back within seconds, and losing a BATTLE feels like being splashed rather than shot.
Ink is ammunition, movement and score
The clever part is that one resource does three jobs at once. INK is what you fire. It is also your fuel: dive into your own COLOR as a SQUID and you refill while moving fast and nearly invisibly. And it is the scoreboard. Every shot is simultaneously an attack, a road for you to travel on and a point on the board.
The weapons follow from that. A ROLLER covers enormous areas but has no reach; a CHARGER hits precisely from a distance while painting almost nothing; a BRUSH lets you sprint and scribble. A TEAM of four identical weapons is a team that is about to lose.
Everyone plays the same maps at the same time
Splatoon does something most online games would never dare: it refuses to let you choose the map. A couple of arenas are in rotation at any moment, they change every few hours, and that is what everybody in the world is playing.
It sounds restrictive and works beautifully. Nobody gets stuck waiting for a match on an unpopular stage, every player learns the same ground at the same time, and a WEAPON that is brilliant this afternoon may be useless tonight — so the whole community adapts together instead of splintering into a hundred half-empty lobbies.
The whole world votes
Every so often NINTENDO runs a SPLATFEST. The entire player base is asked something faintly ridiculous — ketchup or mayonnaise, cake or ice cream — picks a side, puts on a themed shirt, and plays all weekend while the results are counted globally.
It is a very silly way to make an online game feel like an event, and the outcomes get argued about for months afterwards. Combined with the GEAR, the GOGGLES and a soundtrack performed by fictional squid pop bands, it is why Splatoon feels less like a shooter and more like a music festival where everyone has been handed a paint gun.