I look at D&D as a specialized toolbox. It's nowhere near as generic as, say, Savage Worlds or the Fate system, but, by and large, because it's a toolbox, you can do a lot of different things with it. But, because it is a specialized toolbox, it's not going to be terribly useful outside of a certain set of jobs.
Urgh, I think I just touched that metaphor in many bad ways.
My point is, there's nothing wrong with suggestions within the rules for how something might look or work. That's fine. Where I have a problem is when those suggestions take on the weight of canon and must never be changed. We cannot do X because of some established bit of text from fifteen or twenty or thirty years ago.
At one time, racial tensions were baked directly into the rules. You had a chart in the 1e PHB that told you, specifically, how various races viewed each other. To me, that's too much. That's setting material. I'd much rather start with baseline descriptions of the various player races and let the players figure out how different races feel about each other. If one DM wants elves and dwarves to get along, that should be fine. If kobolds marry gnomes in my world, that should be fine.
Telling me that race X is some way, and then enforcing that in all subsequent publications, and refusing to accept any revision or re-visitation is not fine. IMO.