Okay, I have a weird one that tends to apply to games where you explore civilized or once-civilized areas:
Being able to jump on top of guard rails.
Like, whenever you see a rail for a catwalk or balcony or staircase, being able to just hop up and stand on top of it. The ironic thing is that this is hilariously unrealistic; maintaining your balance on something so narrow would be difficult enough, but JUMPING BODILY ONTO IT should only end in disaster. But working under the assumption that my character just climbed up works much better, and I find that on a subtle level, it's the sort of thing that makes me feel like I'm physically interacting with the world.
A good example of the difference this makes is the Half-Life games. In most of them, you can jump up on top of any rail. But Portal 2 didn't allow this. It was a tiny thing, but it made a noticeable difference for me in terms of taking away the illusion of freedom, which has always been important to my Half-Life experience: the games have never been open-world, but they feel like they are. Even walking along a narrow catwalk, where the game was obviously railroading me onto a linear path, it didn't feel that way as long as I could, if I so wished, hop the rail and fall to my doom. I never did, of course, but knowing that the option was allowed me to forget the fact that, again, I was being railroaded onto a linear path.
But taking that away in Portal 2 did make me feel railroaded onto a linear path. It didn't kill the game for me or anything, and I still had a blast with Portal 2, but it did make a difference, and the Enrichment Center, honestly, came to feel just a little bit less alive.