Well, for one, watching the 2011 anime (maybe the '99 version of the Yorknew City Arc as well for comparison) is a far better choice than reading the manga with the inconsistent art quality of the latter. As for what makes it stand out from the crowd:
Character writing - This is probably its strongest aspect with how many of its characters go against typical perceptions of their archetypes within the demographic or narrative such as having sympathetic and fleshed out villains, a protagonist that can be quite selfish at times, etc. as well as a good focus on the psychological aspects of its characters.
The Power System and Combat - Nen is very acclaimed for being a very fleshed out, intricate and unique take on the typical Ki-based magic system. How each characters' ability works can make many fights far more interesting and reliant on strategy than the linear power-scaling of its contemporaries. It's far from a flawless system with how several elements later in the series end up breaking it to some degree, but it's still one of the best takes on a magic system in the medium.
Theme exploration/philosophy - Togashi draws on many different cultures and outlooks for the general philosophies of his characters and narrative as well as the overarching themes of each arc. For example, the heavy use of Christian symbolism in the Yorknew Arc and how religious parallels or inversions have a major impact on the characterisation of several characters, the structure of things such as the Phantom Troupe, etc. or how the Chimera Ant Arc has a heavy focus on things such as existentialism, the debate on whether collectivism or individualism is more beneficial and the dark impact of humanity on the Earth.
The structure - As early on as the tournament bracket in the Hunter Exam, it's made clear the series' structure is far from the typical Shonen fare with it being very rare you have things such as a 1 V 1 confrontation between the protagnist and the arc's villain or the build up to some kind of great climax very rarely being done. This can be a hit or miss element due to the rammifications of subverting a narrative though, with the end of the Yorknew and Chimera Ant Arcs being a very well done subversion of expectations whereas something like the same in the Election Arc being absolutely terrible.
Overall, HxH has quite a few elements that place it significantly above the standard Shonen and warrant checking it out in some form, though it's far from the masterpiece many claim it as with things such as how Greed Island contradicts key parts of its world building or the huge plot conveniences in the Election Arc making it far from the peak of Shonen.