Imagine this situation: you just started reading that one manga and thus far, it seems like your average ninja-themed Shounen. And then, by the second book, you're confronted by one of the main characters' inevitable death. Just. Like. That.
Nabari no Ou starts out as one of those cliché but promising action manga. However, after the first volume the action becomes a sporadically spread thing. So, if you were looking for showy Juustu and flying kunai, you may go back to Naruto, then.
Yes, I'm saying it straight out- being a fan of fighting scenes (though I do like them a little less drawn out) myself, it does feel kind of lacking in that aspect. But every here and then, a few brawls happen. It's just that being a ninja isn't all that important to the storyline at all.
So, without fights, what is so good about this manga? Well, it's the feels.
As I said above, you are quickly confronted with a main character's nearing and inevitable end, and yet you are to watch him get close to others and trying to get his last wish fulfilled. Which is, in my opinion, one of the best and most painful takes on death in the manga I've read up to now.
Overall, the characters are a charming bunch. Even though it seems like there are some really detestable guys around, once things are going down you will find yourself not hating them by the end of the story. It's because their personal backgrounds are tied up so nicely that they all just have reasons.
Which is also why this doesn't have many plot-holes.
Well, it's hard to not spoil too much here, but let me put it this way: there's been three times that I've cried waterfalls over this manga. It pulls your heart-strings just right and if you're in for something else, then give it a shot and feel into the story.