For me, a game’s replayability comes down to how good the game loop is, how much friction there is between enjoying that loop and how well the game is communicating the player’s mastery or progress of that loop. Some examples: Splatoon’s core loop comes down to shooting and traversal. Shoot ink to take out enemies and spray the floor to move around in the level. A newbie can learn all the mechanics they’ll need and understand it in the tutorial and they won’t need to worry about getting tripped up by new weapons. And basically, the shooting the moving is the same in online and offline. But for me, what made it different than a lot of other shooters is that the online matches are brief, bit sized explosions of joy, misery, elation and defeat. The game is good at letting players know what has to be done to meet a win condition. so even if one loses, it doesn’t stick for long as a new match starts. So, win or lose, the core loop of gameplay remains the same and remains fun, making it replayable.
Another example I got is Xenoblade Chronicles. Even for a JRPG, the MMO-inspired mechanics make this a daunting game to get into and even after I beat this a couple of times, I do love playing it over and over because of how well each mechanics flows into one another, even though they can exist independently from one another. In any session, I can grind XP from the very generous combat system (making it real time helps). Or my random wandering can help me get items I can use for a sidequest. The lack of friction comes in from how much land I can explore and how easy I can access parts of the world that I’m clearly underleveled for.
Replayability always come down to how much fun doing something is and how much work it takes to get to the fun parts. If the fun comes from the actual playing, I know it’s a keeper. Bayonetta is a game I can beat and even though I’m not trying to get Pure Platinums, I love seeing how better I’ve gotten at each battle.
And replayability for me doesn’t come from unlocking something after completing some inane challenge where the difficulty spikes for no reason. Trophies/achievements don’t help either. If the moment to moment is fun and there isn’t a huge gap between each moment, I will replay that game until the game breaks