As OP, I should follow up on what I think.
To my mind, the clearest is the difference in fantasy between sources like Lord of the Rings or Narnia, versus sources like Conan or Game of Thrones. In gaming, I see more historically-inspired settings like Harn or Tekumel where there is a distinct culture with values different than modern-day. On the other hand, most D&D is more like the former.
There's nothing wrong with either, but they're not very compatible, and some people may want more the feel of one or the other. It's not so much a problem as a dial that I try to be aware of.
I haven't run much realistic historical settings. I have run the very 1970s sword & sorcery Wilderlands, and slavery has been a bit of an issue, with PCs going all out to abolish slavery, a very marginal ideological position in the setting. I don't recall anything else being such a trigger - serfdom certainly is not!
For us, one that comes up quite a bit as something that gets only a nod is the whole noble/commoner thing. Yeah, it's there. It's in the backdrop. Most of the people in the world deal with it. Players in my game largely don't want to play in a game where there is effectively no social mobility along those lines. So we tend not to, or assume that adventurers are one of the rare exceptions. A game where adventurers have some social status and responsibilities to go with it? They are all over that. A game where adventurers are hobos, murder type or otherwise, trying to live below the social radar? They want nothing to do with it. Long as I get everyone on board on the issue, the rest of the world can be as dark or epic as we want on everything else, no one will even blink. Though admittedly, there's only so far I'd want to push the dark part. So not sure I know where the edge truly is.
Yeah, that's exactly the sort of thing that I was thinking of. I can enjoy grimdark -- but when I'm playing more escapist, light-hearted games, I'm not so interested in the dark side of history.
I can do some stuff by just shifting spotlight. In my vikings campaign, I could say that theoretically rape and atrocities were happening during the various raids and battles, but we weren't going to have them highlighted. Likewise, there were slaves taken on raids and around in the households, but again they weren't the focus. That approach only goes so far, though.
When I'm playing something like Middle Earth, I want the heroes to be more straightforwardly good - taking after the books. Yes, it's often anachronistic (as it is with many things like clocks and fireworks) - but it's within suspension of disbelief.