What are game play elements that keep players invested in the game?

For me it's world exploration and neverending grind of my character's strength growth. Pvp, pve, quests, etc - that's all good, but that's just plain gameplay. I've got a ton of that in any other game. But if the world is interesting to explore and I can grind the game for hours to get a +0.1% power for my char - now that's what would hold me in the game.

Immersion and 'short-term nostalgia'.

When you say 'keep players invested' I assume you're not talking about the player type that logs in every day for a social reason, especially not in this era of Discord where they could log into their Guild's chat without ever logging into the game.

And probably can't be talking about the feelings that most combat parts of games bring, since those games are a mental 'slot' that is normally replaceable by some other game, which means it must offer unique combat or opponents, which is currently unlikely.

So immersion, because it 'creates a space within the mind that has its own structure' and 'nostalgia', which is somewhat related. Once that 'space' has been created, one gets the urge to return to it. If the game is consistent, then the logical player can think through whether or not logging into the game will return them to that space or not ('dead' games, for example, have usually lost this over a long time - often through changes meant to make things 'easier', but shallow games tend to 'run out of it' quite early).

I recognize that these aren't 'game play elements' but that's because it really could be anything as long as it fulfills these conditions, and MMOs are among the least limited. All the Developer has to do is make sure that whatever a player is experiencing for the first few hundred hours doesn't 'fizzle out' too much beyond that, and that the worldbuild/play loop manages to hold a 'special place in the player's heart' or similar, so that they occasionally yearn for it.

"Yearning to return" + "nothing quite like it" + "ability to reason that the game is still equal to the experience you wish to return to" = "Retention"

MMOs have the problem of needing to have all that stuff for a larger critical number of people than most other games, but every game has their specific challenges in this equation.