Much modern fantasy falls apart because the characters do not ring true to their universe--they don't think like pre-industrial people, they think like middle class Americans.Worse than that, even.
They think like upper middle class college-ed liberal Americans.
Some particular pet peeves.
Few or no gender roles. Look folks, in a pre-modern economy there are inescapably logical reasons for men and women being treated differently. It isn't just that the men are being mean. And sexually liberated women in such a society get knocked up and punished with a child, which puts a real crimp in her non-traditional lifestyle.
An extreme variant of this is the female warrior/mercenary. An exceptionally capable woman warrior might go toe to toe with an average male warrior. She meets a better male warrior and she's toast. If you insist on the female warrior, at least bother to provide some backstory for why she is able to compete with men. Don't just ignore the physical differences between men and women.
All premodern societies we know of are deeply involved in religion and/or superstition. I realize modern readers are uncomfortable with this, but again bother to provide some sort of reason why your pre-modern society isn't religious.
One of Tolkien's major flaws, IMO. Not a trace of religion in hobbit society, and not much elsewhere. In LOTR, that is. More in Silmarillion.
I find much modern fantasy absurd--as you noted, ignoring gender roles is an obvious problem. Wishful thinking does not make things plausible.
Few women have the upper body strength of men. They just don't. Women frequently adore horses--I certainly do--but there have been few top class women jockeys. Julie Krone was very, very good, but she was a rarity because as good as so many women are working with horses, they generally lack the upper body strength to manage 1200 pounds of Thoroughbred running 35 mph.
Americans have the curious notion that class structures don't matter in such a society. This leads to absurdities such as The Peasant Boy two weeks removed from digging turnips being completely comfortable chatting with the ruling class.
Not only would this never happen--they would likely speak very different forms of the same language--but anyone at the bottom of the societal pyramid would have had the notion of his "place" imprinted so strongly that he would not freely speak to his "betters".
Tolkien's lack of religion is a striking omission. Perhaps he did this because of his own strong faith; depiction of any other kind of religion motivating his characters might have been awkward for him.
A few decades back, fantasy protagonists grew up or became stronger by winning through to a goal by way of hard work and determination. What I find appalling is the protagonist who achieves because he was born with a special gift or destiny that he did not earn. He's Just Wonderful, and all good things come his way without his breaking a sweat. This appeals to teens unprepared to work for anything, but it's a cheat. Mercedes Lackey built a career on such protagonists.