At first glance, this manga might seem to be just another shounen battle manga but with a lot more blood. I cannot fully deny that claim but I can at least say that this manga tries to address the political, social and economic factors of war. This fact alone is what elevates Kingdom above all the typical shounen drivel to me. Numbers matter, politics matter, strategy and tactics matter, geography matters, history matters, there are important characters besides the protagonist, his friends, and his enemies. Finally! In a sea of "war" series where war is used mostly as an interesting back-drop to the main characters' personal problems, finally there is a manga that focuses on the actual war!
If you want to give this manga a chance, wait until Shin's first campaign (from ~ch48 to ~ch73) to decide whether or not to keep it. The first 47ish chapters are basically a prologue. It introduces the setting, the main characters and their motivations and provides enough intrigue and combat to keep you from getting bored. However, the true showpiece of this manga: war, begins with Shin's first campaign. I think Yotanwha (mountain tribe leader in the manga) said it best: it's a "carnival of blood." And as Shin (as a rank and file foot soldier) is tossed to and fro in a massive battle, it truly feels that way. Each of the different parts of the battle feels like a different, perverted ride in Murder World. Have you ever wondered what it was like to be in the first row of an human wave infantry charge into a wall of shields and spears? How about being in the path of huge war chariots as they plow through your comrades like a lawn mower? What about being part of a desperate charge at a group of archers that are killing everyone around you? If Shin's first campaign doesn't grab you, then nothing in this manga will.
To back up an earlier point: numbers matter in this manga and this is one of the primary characteristics that separate this from so many other shounen manga. Granted, there are plenty of hot-blooded duels (of both the mental and physical variety) between officers, but there are no one-man-armies except for one guy (and screw him). The manga makes a point in saying that although the general may head the charge, the strength of the charge comes from the men who are charging with him. I still find that personal strength holds undue weight on the battlefield but the mere fact that people besides the named characters effect wars is such a refreshing step up from other "war" series that I can hardly complain.
The fact that numbers matter also adds some new dimensions to the traditional shounen bag. Most shounen battle manga have only one way of measuring effectiveness in battle: combat power. If a character's combat power is higher than their opponent's, (or they have a "final form" that is more powerful) they win. Simple. In this manga, at least, there are three ways of measuring effectiveness in battle: strength, knowledge, and leadership. Thus, a character with high knowledge but low strength might be able to outsmart a physically stronger opponent and defeat them (and this happens often) or vice versa (this also happens often). Simply having more than one way to measure battle effectiveness adds much needed variety in the battles. It also gives Kingdom a leg up on other shounen battle manga where you basically wait until everyone is out of final forms and such to find out the victor.
As for the characters, they are fairly good. It's particularly refreshing to have grown men as important characters, even if the teenagers steal the show. Shin is your typical, hot-blooded, big-dreaming, shounen protagonist with the exception that he's okay with killing people. Maybe his character type is over-used, but it does the trick for me. He's also incredibly manly. Kingdom also has a secondary protagonist in Sei. This is the guy who eventually unites all of China. The author wanted to make Sei the main protagonist but he was afraid that Sei wouldn't appeal to mainstream audiences. I don't know if the author does Sei justice, but he's at least not like other characters. He tends to play Spock to Shin's Kirk (as rare as they interact) but he is also extremely motivated and holds a rather unique position in the manga. Also, I don't want to spoil too much, but there's a female character in this manga who is pretty kick-ass. I especially like that the kick-ass female character isn't out to avenge her father (or something) for once.
The story is good. It features plenty of political intrigue to you keep you engaged if Shin's story doesn't grab you. The art is also good. You don't notice it in the page-to-page manga, but the spreads are amazing and brutal. Also, maps! I actually know what's going on in the battles! Amazing!
TL;DR: What appears to be yet another shounen manga with blood is actually much more. Give it a try.