Like Reality Unless Noted/Quotes

Tabletop RPGs are a medium of virtual life-experience. The tropes of storytelling do not apply. Real life's "tropes" apply, even when you would not think so. You literally have to warp the rules of the game to make trope-based decisions happen because they are not viable course of action in the real world, and by default the environment of the game--no matter what genre it claims to be--is the real world with only those exceptions that the game explicitly postulates.

— Bradford Walker, My Life as a Gamer: The Benefits of the Medium [1]

Here’s the problem. From a nuts and bolts story telling perspective, your readers are going to assume that everything in your book is similar to the world they currently live in, until demonstrated otherwise. Unless you say that in the future everybody has been genetically modified to have 3 legs, they are going to assume that all your human characters have two legs. If you are going to demonstrate that something is different, then there needs to be a reason for it. So if you say all humans have 3 legs, but it doesn’t play into the story at all, then why bother? And every time you change something to be different from the expected, there had better be a reason for it or you will quickly just annoy your reader.
Reading sci-fi like that grows tiresome. It is like listening to an inexperienced little kid saying “Look, I can do THIS! And now I can do THIS! Isn’t that the neatest thing EVAR!?” And your response is “Yeah, yeah, that’s special…” when you’re really bored as shit and don’t care how tall their Lego tower is the 50th time.

— Ending Binary Gender in Fiction, or How to Murder Your Writing Career by Larry F. Correia