I can't speak for others, but I love HPMoR. I honestly believe it's one of the best pieces of fiction I've ever read, so I'll try to describe my own reasons.

1. Tropes and Plot Devices: I've read a lot of sci-fi and fantasy and HPMoR avoids a lot of the downfalls of the genre such as dei ex machina, whiny/angsty heroes, and phleboninum/unobtainium. Eliezer is familiar enough with common tropes that he does a great job of applying them in the right contexts, subverting them interestingly, and sometimes calling them out and making fun of them directly.

2. The First Law of Rationalist Fiction: Roughly, that characters should succeed by thinking in understandable, imitable ways, not by inexplicable powers or opaque "bursts of insight" that don't really explain anything. After hearing this ideal stated outright and seeing it in practice, a lot of other fiction I've read (and, unfortunately, written) seems a lot less satisfying. Eliezer does a fantastic job at giving a look into the characters' minds and letting you follow their thought patterns. This makes it even more satisfying when they succeed and even more crushing when they fail.

3. Application of the Sequences: As someone who had read the sequences more than once before reading HPMoR, it was both fun and enlightening to see the ideas put into practice in a high-stakes scenario. As someone who was familiar with the ideas beforehand, I got to smile and "get the reference," yet also be surprised by the application. I honestly believe that I've started applying rationality to my real life more after reading MOR and seeing examples of how to do it.

4. Balance of Power: I think MOR does an excellent job of maintaining a balance of power between the characters. At times, you think your protagonist is horribly weak and at other times, they appear very strong, though perhaps in a different way from the other characters. The conflict is never one-sided.

5. The Characters: The characters are dynamic, multi-faceted, and sometimes morally ambiguous. You never fully understand their goals or motives, but they are there and they are consistent. Even the "dark" characters have certain insights to share that sound "bad", but are nonetheless seductive and often hard to argue with.

6. The World and the Lore: As a long-time Harry Potter fan, seeing this kind of adaptation of the world is just fun. It's a rich world in which to set a story and Eliezer does a great job expanding the lore, making some of it grittier, and making a lot of it much deeper and more mysterious. While he had a great platform to start from, the world-building is fantastic.

Overall, the story is incredibly entertaining and fun to read. It has a lot more to offer than most fiction out there. I love it.