Perhaps one of the strengths of the series was that it had a bit of something for everybody. At times it was just a "kids' cartoon" with wackiness and simple, sometimes even crass, humor. At other times it had spectacular battles of epic proportions.
At times, however, it could be really deep and touching. After all, it started as the story of an orphan boy who was shunned and avoided by almost everybody in his town, for no reason of his own (the actual reason being that an extremely powerful and dangerous evil spirit had been sealed within him by his father, to save the town from its rampage; the other people feared the spirit and the person carrying it, perhaps because they considered him a ticking bomb that could either release the spirit or go himself into an unstoppable rampage at any moment, powered by the spirit.)
The series, at many points, thus deals with quite deep and touching subjects of loneliness, prejudice, friendship, and not giving up, and can be quite sentimental at times. This quote is quite descriptive of that (Naruto speaking to another person who likewise had a spirit of the same kind sealed inside him):
It's almost unbearable, isn't it… the feeling of being all alone. I know that feeling; I've been there in that dark and lonely place, but now there are others, other people who mean a lot to me. I care more about them than I do myself, and I won't let anyone hurt them. That's why I'll never give up. I will stop you, even if I have to kill you! They saved me from myself. They rescued me from my loneliness. They were the first to accept me for who I am. They're my friends.But then the series just seemed to fall more and more into obscurity. Less and less people were interested in it, and people were talking about it less and less.
The series overall can be divided into two parts, the original Naruto, and its "continuation" series Naruto Shippuuden (which happens something like 5 or 6 years after the original series ends, now with an older teenager Naruto.)
The popularity of the anime was relatively strong all the way through the first part. Up until the absolutely infamous filler flood.
You see, one problem with anime that's adapted from a long-running, still on-going manga is that the production of the anime tends to be much faster than the manga (which might sound quite unintuitive, given how much more work there is in creating an animated series compared to a simple comic book, but that's just how incredibly efficient and productive Japan is at producing anime), and it often happens that the anime "catches up" with the manga, and there aren't many, or any, new manga books to adapt. In some cases the anime is simply ended there, even though the manga eventually goes on, sometimes for quite long. Othertimes the anime starts creating original episodes, even entire story arcs, to give the manga creator time to publish more books.
The Naruto anime series opted for creating filler episodes (sometimes consisting of mini-arcs of a few episodes, sometimes with individual episodes pretty much not connected to anything). A lot of filler episodes. A whopping 84 of them.
That's more filler than most anime series (or any TV series for that matter) have episodes in total, even if they run for several seasons. With one episode being produced a week, that meant almost two years of nothing but filler that did not advance the main storyline. And the quality (both in terms of story and visuals) was often significantly poorer. It was absolutely insane.
Not surprisingly, this pushed away quite a significant portion of the fanbase. They just got tired of the filler episodes, and moved to other, more interesting series. Even half a year of filler would have been quite a lot, but almost two years... It just was too much.
Then, finally, the actual episodes adapting the manga resumed, with Naruto Shippuuden. It started quite strong, with very interesting episodes and story arcs, and it seemed to have been gotten back in form. The series dealt with almost nothing but the main story arc (with perhaps just a few individual episodes dealing with something else). And this lasted for quite many episodes.
Then the flashback arcs started. At some point, the main storyarc would just be interrupted, for no rhyme or reason, by starting a completely unrelated, often quite long, flashback arc, dealing with some aspect of the past of the characters or the village. These flashback arcs could span as many as even 20 episodes, making them really, really long (almost half a year long). Then, when the arc was over, the main storyarc would just resume from where it left. There was usually little to no connection between what had been happening in the main story arc, and the flashback. It was just inserted there seemingly arbitrarily.
And the worst part of it was that the flashback arcs were usually much longer than the stretches of the main story arc that were shown between them. They could eg. show a 15-episode flashback, then something like 5 to 10 episodes of the main arc, and then another 20-episode flashback (that had basically nothing to do with anything).
And the thing is, while the production quality of these flashback arcs were ok, they were boring and felt like artificially stretched. They felt completely full of useless filler.
I have never seen this done with any other anime series (or any TV series for that matter). I don't know if these flashback arcs were adapted from the manga, or whether they were, once again, created solely for the anime, to slow down the main story arc, but it doesn't really matter. They were absolute filler, and made the pacing of the main story just horrible.
This kind of pacing just kills interest in a series. Which is a shame, really, because it's not a bad series. Watching it from beginning to end, with all filler and flashback episodes removed, would make it a quite good series.