Story: What's interesting about the story is that the plot itself is pretty original, at least to an extent. There's ayakashi (demons) which isn't exactly original, but I hesitate to call it cliche because it's more of a subgenre. However instead of flashy swords or guns, the characters use boxes called "kekkai's" which trap subjects and then destorys them, kind of in an explosive kind of way. What makes it more interesting is the ways the characters use them as not only destructive weapons but tactical ones as well, and they continue to come up with new ways to use them which might have you thing, "huh, I wouldn't have thought of that." Of course, the kekkai's are only used by the kekkaishi users, and the further you go into the story the more you discover more powers and whatnot, which includes a lot of really unique ideas. The ayakashi, too, can have some really neat traits.
Kekkaishi begins like most shounen manga: first are missions and then an overall plot. What's good about kekkaishi is that it stays true to the original plot, and while it appears to deviate at points it always connects back to the origins. There are so many events that happen which you wont think are significant later on, and then they are!
Art: I think art is pretty subjective, but I think that most people will come to like it after a few chapters regardless of what they think originally. If you're reading shounen for the bishounens, this probably isn't for you. However they aren't unattractive by any means, the style is just a little different from your average shounen. Some of the sceneries are very well drawn, and the action scenes are very comprehensible, which I find sort of rare and is definitely a plus.
Character: One thing that everyone looks for in character is the development, which is plentiful in Kekkaishi. The biggest example is the main character, Yoshimori. He starts of as headstrong and overpowered, but has no idea how to use said power so is fairly useless. This isn't a very new idea, but what I like about his development is that while he he changes drastically throughout the manga, he's still the same character. He's very entertaining and once you get past some of his more annoying traits, he's very likeable. After all, all characters have their faults!
The side characters go under some major development as well, another rare trait of shounen manga. And I don't just mean the major side characters (in this case, Tokine and Masamori) but also in others. Tokine is the main female lead, and she's pretty well done. She's the brains of the team, but while her power of strength isn't great, she gains power that isn't for strength but for other purposes, so she's isn't useless (I could name a lot of useless female shounen characters.) Masamori is Yoshimori's older brother, and he plays a much bigger part than he appears. He's a character you might just hate to love at some parts!
The character of the antagonists are also pretty impressive. They undergo development and have good backgrounds to them!
Enjoyment: This is where the "9" ranking comes into play for me. Despite the cliches and some of the other problems of this manga, I really enjoyed it. However this could change for different people. If you like "all action all the time" then you might get a little bored at some points, but there is a fair amount of it. However a lot of it focus' on characters and story, so some might not be able to get through the parts where there isn't a battle and more talking/etc. I'm personally pretty okay with this and don't mind breaks from the action. What's great is that it isn't confusing like so many other shounen mangas which have so many names and not a face for half of them. No named character is insignificant (to my remembrance) so you'll often be able to attach the name to the face and the background of the character. Also, the plot has a lot of points and gets messy at the end, but even then it's comprehensible who is doing what and where.
Overall, I gave it a 9 because I couldn't put it down (and I read the whole thing twice.) but I can't say it's not without its cliches, which is really the only downfall I can see.
Highly recommended for anyone looking for a good shounen that's not Naruto/Bleach/One Piece/Etc.
