OT: Frankly put, the series simply has such insane quality-inconsistencies that most people get fed up with it.
I honestly loved the beginning of the series (pre-timeskip). The whole idea of magical ninjas was silly, but once I got over that and learned to simply consider them supersoldiers with a ninja gimmick it all made sense. The Zabuza arc was pretty good, and the following arcs were awesome as well. The series didn't shy away from killing important characters if the plot demanded it, Naruto actually grew and the side-characters were also pretty interesting.
Then Shippuden came along, and everything went to shit. Not all at once, mind you, but it certainly became a very long downwards spiral afterwards. The series was still pretty decent even after the time-skip for a while, but then the four cancers of the series slowly began to rear their ugly heads:
-Sasuke: The guy was interesting pre-timeskip. He was a genius rival who took a dark path for the sake of revenge, and his gimmicks were amusing at this point. Jump to post-timeskip and he turns into a pompous, borderline psychotic asshole who makes decisions at random, his eye-powers make him completely broken to the point where it just becomes ridiculous (in retrospect, he is like a rival NPC from a D&D game, where he would automatically gain power levels for no reason whenever Naruto gets stronger just so that the rivalry wouldn't get out of balance), and worst of all, Kishimoto made entire arcs focus just on him because he was a fangirl favorite!
-Sakura and the stupid love-triangles. This issue had its ups and downs but it mainly boils down to this: Naruto has a crush on Sakura for no apparent reason beside that she is pretty even though she is a useless idiot, Sakura has a crush on Sasuke even though the guy is an unrepentant evil asshole who tried to kill her on more than one occasion, Hinata has a crush on Naruto and keeps holding out for him even though Naruto is an insensitive asshole who refuses to acknowledge her in any way and then there's Rock Lee/Sakura and Ino/Sasuke and the glasses chick who has a crush on Sasuke and the other crushes and whatnot... and let's not even get into the crack fan-pairings!
In short, Kishimoto created this love-dodecahedron or whatever way back in the very beginning of the series and refuses to advance the relationships because shippers are a strong demographic that will keep buying his stuff just so they can see whether their preferred pairing will occur, all the while the actual characters suffer for it both in-story and in the characterization department.
-The goddamn Uchiha clan and their goddamn magical eyeballs, with Sasuke at the helm. The Sharingan are not only completely broken, but they are all over the goddamn place even though the clan was supposed to be dead, yet ALL the biggest villains in the series are either Uchiha or use their eyes/their derivatives often in ways that make no goddamn sense. How the hell does a magical eyeball create a huge magical armor/giant/whatever!? Or turn back time!? Or create alternate spaces and unquenchable fire and yada-yada-yada?!
And if that wasn't enough, the Sharingan became an all-purpose plot-fixing glue. Why is this character so powerful? Sharingan. Where did the rinnegan come from? Sharingan. Why are all the Uchiha evil? Sharingan! Everything in the entire series revolves around the fricking magical eyeballs to the point of utter ridiculousness.
-The anticlimaxes. This became a staple of the series in the past few years in particular and it was the ultimate reason why I stopped following the manga (I only know about the recent development because I listen to the Weekly Manga Recap podcast), namely the end of the Pain/Attack on Konoha arc. The conclusion of that particular arc was such a ridiculous cop-out, especially after the great lead-up, that I completely lost my faith in the series, and as far as I know it never really got any better afterwards.
Also, the flashbacks. I would say half of the last arc was made of flashbacks, which is pretty insane taken that said arc has been running for more than two years...
Add to that the insane number of filler episodes in the anime and how Kishimoto apparently started to give less and less of a damn about his writing and his artwork in particular with each passing year and you can see how so many people are fed up with the series...
