The main reasons I like it are the character's, their development, their interactions with each other, the concepts of the HxH world, and the endless possibilities of Nen.
The character's seemed to be so unique and fresh to me when I 1st. saw the series. While most of them fit into generic archetypes, their unique character designs and backgrounds fascinated me. But more importantly I liked their development and interactions.
The character development and interactions were well-done, IMO. You get to see the main characters progress and grow, and base all their actions not only on the present but from their past experiences as well. Also, the minor characters play a very interesting role too. Each, yet again, have unique designs and backgrounds. Some of them are explained while others leave you with tons of questions. But I just think of that as part of the fun LOL.
The HxH world itself is awsome too. The scenery in the series was varied and great IMO. Each area had a unique feel to it, and the way the characters acted in them made the world come alive. I really don't know how else to explain it, but HxH just pulled me into its world and it's a fun ride

Now Nen is just about my favorite thing about HxH. There are just so many possibilites with nen, but there are no fool-proof trump card techniques with it. You cultivate the skill that you have into what best suits you and you go with it. Sure there are better and worse nen-users, but there are no "secret techniques" or "ascensions" possible with nen that assure you victory. You just hope that you can outsmart your opponent or you bank on you're style of nen (and the way you cultivated it) being superior to your opponents' style. So it's just plain 'ole hardwork, smarts, determination and luck required to survive in the world of nen, not just plot-contrivances. It also makes for some interesting battles.
I like the manga version of HxH a slight bit more than the anime. Mainly because I like reading exactly what Togashi had in mind for HxH and getting the full story. I also like it because the manga is going far beyond Greed Island (though it's currently at a stand-still in the Chimera Ant arc for about 5 months now

Anywho, what I wrote above are just a few of the reasons I like HxH so much. I could go on and on, but I'd hate to bore y'all

- RalphVboy