I really like this question, because it is not something I’ve previously given much thought to. Here is what I’ve come up with, in no particular order:

1. Dense atmosphere. There’s a lot I could talk about here, but suffice it to say that the game needs to have Ｔｈｅ Ｖｉｂｅｓ.
2. High interactivity. I’m spending most of my time engaged with the game world, as opposed to transitional elements like menus, lobbies, or cutscenes.
3. Low friction. This doesn’t necessarily mean no challenges, but I shouldn’t have to feel challenged if I’m not up for it.
4. No pressure to advance. I can just dick around for hours at a time for any reason, without consequence.