I think the key to making me totally fall in love with a series is humor... but unexpected humor. Hunter X Hunter and Naruto aren't comedy series, but both can make me laugh in such great ways! And it's all the better because I'm not waiting to be made to laugh.
Usually I don't like main characters in series like this one -- they're usually annoyingly wide-eyed (I don't mean literally so, this is anime after all!), perky, go-geters, but in the case of Hunter, an exception has been made. While he isn't my favorite character, I can't help but like Gon. (Go go gadget! Fishing pole!)
While he's not got the right physical look, my favorite character would usually be Killua's type. (I still snicker/groan at an assassin named Kill*. I wonder if the pun is in there on purpose? Or if that's just a normal Japanese name?) I like him a lot, but I can't say he's my lone favorite. Somehow Leorio has snuck his way into my heart. (He secured his place there with all the Leorio/Pietoro stuff. I wonder if they'll later explain more about poor Pietoro, or if the writers just put him in there for us slashy fangirls?)
Oh! I guess I should explain the incest thing, huh? Some of the voice actors for Hunter also do Naruto and Prince of Tennis. I'm noticing a number of other similarities as well, in characterization (Leorio strikes me as Momo-ish so often!) to plot points (some parts of these Hunter tests remind me a lot of the Naruto testing), but I'm sure that's all chance.
Oh well! Back to watching!