Dark fantasy can actually be a masterpiece if the fantasy story is written to explore the dark side of humanity and find human meaning in suffering. The stories uphold human values... and teach readers to be 'human'. Examples:
- Fullmetal Alchemist: Brotherhood asks: Can human sacrifice be justified for the sake of truth or progress?
- A Song of Ice and Fire (Game of Thrones) asks: Can power be attained without losing humanity?
- And Claymore asks: What is the price of being an instrument of justice in a merciless world?
- And let's not forget Attack on Titan, which asks: Is true freedom worth fighting for even if it means destroying the world to achieve it?
Unfortunately, most dark fantasy descends into total nihilism, losing the meaning of humanity, descending into despair... and even exploiting suffering for entertainment. One branch of dark fantasy even calls itself "grimdark," emphasizing moral decay, an amoral world, without a glimmer of light in the story.
I've always been confused by the term Dark Fantasy, I know it's a genre or theme of sorts, but I could never really put a thumb on what it meant exactly. I've wanted to write a novel with it, but since I couldn't define what it was, it was kind of hard to put it to words. So, seeing proper examples with a breakdown of what the theme was, is really helpful, so thank you!
Now, to answer the post...
I do like well-written dark fantasy. I watched full metal alchemist, the first version, not the second one. Berserk I think counts, both the manga and the old anime, the one from the 90's. As for Game of Thrones, well, I hated the books. A friend got me them, and I hated them, but I liked the television show, until the last season. Dragon Lance books may count as well, and you don't see them mentioned much.
As for what I like and dislike about Dark Fantasy, I like the themes. That it doesn't always go well for the main character, or more specifically, it felt like the world didn't necessarily bend over backwards to accommodate the character. I like that they can posit decent questions and see them played out.
As for dislike, well, like a previous poster said, a lot of Dark Fantasy tends to over focus on the whole death and gore without any real message or purpose. Too many mistake style for substance, and sometimes, they can posit a great question and fall flat near the end.
As a reader what do I expect from a Dark Fantasy? Well, I mean, it's hard. The term itself has lost meaning because it's used so much in the wrong context that when I hear someone describe something as Dark Fantasy, I don't really know what to expect. Even having read proper Dark Fantasy novels, it can be hard to distill exactly what makes them a 'Dark Fantasy'. So, yeah, I don't really expect anything when someone describes something as Dark Fantasy.
Hopefully this answers your questions.