Generally speaking : Footsteps in a videogame's snow = Pure bliss.
Back then I was impressed by songs in games (NOLF or Portal). In NOLF, I seem to remember hearing a Rina Ketty or am Edith Piaf song, in some shed. I also found an unused Piaf song in the game's files. These seem so useless to the game, but so adorable. Nowadays I think that games production became so movie-like that title songs (or in-game songs, in GTA-like car radios) don't impress anyone anymore. For me it meant games were becoming something different.
I'm a big fan of gratuitousness in general. Cathedrals having been build with scupltures that are out of sight of the public is a thing that touches me (okay, I assume they made that for a god who sees everything, or something, but still). Having bought a game in a second-hand shop and seeing that the previous owner had included their handnotes and such, without knowing who would get that, was a wonderful little moment. I love all the "you didn't even need to do that" in life. Actions with no gain, for the beauty of it. Art, I guess ? It seeps into videogames.
And open world games offer a lot of opportunities for that. The creation of a huge 3D world, that no player will explore fully. I'm not talking about easter eggs, but simply very well crafted little places. It impresses me when, in games like Watch Dogs or Mad Max, I find a beautiful place, or a beautiful detail, that had been added "just because", in a place unlikely to be visited. It gives me the impression that the world had been crafted with genuine pleasure. I take such details as gratuitous gifts. As if they went beyond the commercial product exchange, and into the realm of... "playing together" with the person in charhe of that bit. Something like that.
Oh also the photographer giggles when you click his ass, in Infogrames' North & South.