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