ART: In the beginning, the art's a bit sketchy and the action's hard to follow. Luckily, however, the art does get better as the chapters get higher, and the new, cleaner lines really helps the series aesthetically.
PLOT: It's there, but it's kind of blurred. It's more of a series about the trials in Naruto's life without a specific plot-line (unless you count becoming the Hokage a plot-line, there isn't one that is there throughout the manga). Nevertheless, this doesn't hurt the series as much as one would think, since the plot morphs as new trials emerge (ex: Orochimaru, Akatsuki, etc).
CHARACTER DEVELOPMENT: Beware of flashbacks. There will be MANY. And you can't forget about the angst. Some people might get annoyed with the fact that almost every single main character in the series has some kind of angst-ridden past, but the fact that the levels of angst vary between individuals helps it from getting TOO clichéd. The only annoying part about the flashbacks is that they take so long to get through chapter-wise (I swear, in the scene at the Valley of the End it was just getting ridiculous) and sometimes interrupt interesting action sequences even though they do fit in with the situations.
TECHNIQUES: The combination of ninja jutsus and element-based attacks is definitely intriguing, and the kinds of techniques found within the series are very creative. Summonings are plentiful as well, and the creatures summoned all have their own personalities, giving the idea another creative twist. However, there are moments when the series has a DBZ-esque way of having techniques (or people) "evolve" (the sharingan, Kyuubi, Orochimaru's cursed seal), which sometimes can become annoying as it gives off a feeling of unlimited power. But there are limits placed upon these powers, so at least the feeling of having unlimited potential is hindered somewhat. The fighting style in the series is very unique thanks to the ninja-style fighting, and it's refreshing when compared to the strength-based attacks of other shounen titles. The strategies detailed in battles are also often both creative and ingenious, giving battle another facet that more mangaka try to stay away from in shounen titles (just look at Shikamaru's battles for examples).
CHARACTERS: Well, Kishimoto-san sure outdid himself with the menagerie of characters in Naruto. You have everything from pervert to homocidal maniac to pedophile (well, they never say he's a pedophile, but it's pretty obvious that he has some of those tendencies). I personally have never found any two characters in the series to be too similar to each other personality-wise, and, considering the number of characters the series boasts, that's saying something.
LENGTH: Yes, it seems long when you look at the number of volumes or chapters. Still, it's difficult to notice how many volumes the series has gone through when you're actually reading it. Ignoring the battle at the Valley of the End (which only seemed long because of the drawn-out flashbacks), the series seems to go by quickly without making things seem drawn out.
As for negatives, I am really disappointed in the number of Naruto's techniques. Sure, he has the Kyuubi and extraordinary stamina, but what use is that when he only has two useful techniques that don't injure him whenever he uses them? His summoning isn't even fine-tuned enough to aid him in a normal battle, and Kage Bunshin can only get him so far. If he learned another USEFUL technique (spoiler warning: yes, he learned his "ultimate technique" for his element, but its use is limited, therefore making it rather useless in battle), then maybe he'd seem a bit more of a balanced fighter. Until then, I can't say that he's advanced any farther than his strength BEFORE he left for two years with Jiraiya.
So, all in all, this manga is very good, but there are a few drawbacks that keep it from getting a 10/10 score, such as originality (there are quite a few original concepts, but it sticks pretty closely to the clichés of shounen manga).