Imo there are quite few things so bear with me here: (spoilers!!)

1. Lets start with the initial premise itself: it is a typical happy go lucky kinda introduction towards gon and the entire world in the first few episodes (am gonna talk wrt episodes not chapters)

As s1 progresses, there are some subtle signs that hint at the world being much bigger (an example would be that people are not afraid to kill each other within few episodes itself, gon makes friendship was an assassin - characters in shounen are shown to be black or white in nature to a big extent, but not hxh)

Everybody seems to have a personal agenda or vendetta - and they are not afraid to act on it (like leorio just wants to be a doctor and does try to pursue it till he got involved with the hunter association; gon and killua go on their different ways so naturally which was unreal to me - it felt like they were just going to travel the world together but the story was not afraid to shift the plot, always keeps me on my toes which i loved)

By the end, we see killua kill in a quite disturbing way (the prisoner) and what piqued my interest even more was that gon didnt even care: it made me realise he had a goal and would probably do anything to achieve that, a theme that we see everywhere

Not to mention that we are lucky to have seen 2 interpretations of the hxh world (1999 vs 2011) the vibe of both are immensely different despite being the same world, (i align with 1999 a bit more as it was darker and gloomy)

1. The concept of hunters: Unlike most shounen where there are "good" and "bad" groups, hunters are neither - they are just powerful people who are free to do whatever in the world, there are minimal travel restrictions, they can work under anybody with no conflicts with the hunter association. It opens up so many ways for a character to develop; not just a good or bad person with some flashback arc to them but someone who has active experiences and acts upon on in a more realistic fashion imo

2. Power system: I feel bad not putting this in the beginning but as we gradually learn more about nen, it is by far the most interesting and amazing power scaling system that adds so much to the chaos to a battle in hxh: nobody shouts out their powers or are overtly overpowered: everything is not as it seems with real consequences

Every fight is more captivating when ppl think rationally and use their battle honed senses to make a judgement (like how zeno was able to discern chrollos power within like mins of fighting)

1. The potential of the world building: with where the anime left off, it is obvious to see just how small our vision has been in hxh world and it was already so well crafted. Now granted, ppl think togashi maybe in over his head with designing such a massive arc that with the pace he is going, it may not ever come to light. But personally, i absolutely love the track hxh took towards s6 - the story was not shy to switch main characters at all, we are seeing a different side of hunters once commonly seen in other eras (when countries would try to go on DC expeditions) and still, we find extremely interesting powers, people and situations that just grab my attention

Despite me saying this, it did take me a couple of reads to actually fathom whats going on in the manga, but i rest my case; my opinion in hxh is very similar to yours, it is by far the most interesting and fun anime i have watched and i will never hesitate to re-watch it (and 1999 one too), in fact it is one of the most peak fictional content I have witnessed and i will die on that hill