A Ninja and A Death God: Naruto: Fillers Are Canon, they are not trash, you are missing a lot of things if you miss them (contains lots of useful information to non-filler fans) (huge spoilers to those who want to watch the fillers).

Most of the people who watch anime are mad about the fillers.

Here's my answer to that

Well actually you guys shouldn't be mad about the existence of fillers.They are canon, and most of the time, they introduce things in fillers first than in manga. Here are some of the important things shown in fillers that shouldn't be missed:

Sora Arc: Matatabi (2-tails) and Yugito was first introduced here, as shown when Hidan and Kakuzu were apparently chasing the 2-tails.

Three Tails Arc: Nuff said, showed how Deidara and Obito captured the jinchuriki-less 3 tails.

Six Tails Arc: Utakata was first shown here, and later shown that he's actually the Jinchuriki of 6-tails. Yeah we all know it's canon as shown in the manga:
Utakata: I know that you already know me Naruto, but I'd introduce myself anyway, I'm Utakata the six-tails' jinchuriki.

More detailed war fillers: One of the most interesting part of the filler is the appearance of Kabuto's foster-brother in the war. He said that he's still waiting for Kabuto despite being gone for a lot of years. He says that Kabuto still has a place to call "home". Another interesting event was when Deidara escaped. In manga, he just suddenly went quiet for no apparent reason. The filler showed why. He escaped and recaptured again then he realized that his "art" is no longer applicable to him. His principle is "art is a blast"(shows in a flash then disappearing, just like fireworks). He used his suicide-bomber jutsu and still revived afterwards. He's disappointed and decided that a better art-principle is appreciating his handsomeness. He then kept quiet afterwards.

Deidara Escaped

Chikara Arc: The appearance of 7-tails cloak is shown here. We know that Naruto skipped 5-tails and went straight to 6-tails then skipped 7-tails and went straight to 8-tails. The filler showed that after 6-tails, the 7-tails assume the form of kyuubi itself then acquires muscles on 8-tails. It also showed that Hidan is finally dead because Kabuto revived him using Edo Tensei. The filler also showed how Kabuto perfected the said jutsu.

Extended Obito flashback- Akatsuki Formation Arc: It was shown in the manga how Obito slowly became a bad guy and how he tried to persuade Yahiko and Nagato. However it was expanded in the filler. It was shown how the Akatsuki expanded from small ragtag band of activists to huge fraternity. It revealed that Akatsuki's principle is the same Hiruzen's: warless conflict solving through pure negotiations, but to a greater extent. Hanzo was shown to be very impressed of Akatsuki. It was Danzo that selfishly, nonsensically and heartlessly corrupted the mind of Hanzo by showing fake bodies and ordering his ANBU to suicide themselves to Akatsuki. Though Akatsuki doesn't want to kill, Root ANBU purposely impaled themselves to the defensive kunais of the Akatsuki. Having a corupted mind, this was the trigger to Hanzo ambushing Yahiko and Nagato by kidnapping Konan. The filler also gave the reason why Nagato used Yahiko as the Tendo Pain: "Yahiko will always be the leader of Akatsuki -Nagato". Oh, and it was Yahiko who created the name Akatsuki after Kyusuke said the a name is becoming a necessity as the group gradually expands.

Kyusuke, the one who said that they need

a name due to their expanding group.

Danzo corrupting Hanzo

Izanami Flashbacks: People made fun of the Izanami's purpose for a long time. People said sarcastically "Izanami is made to prevent the addiction of Izanagi, a jutsu that can only be used twice. lol".
So the filler clarified it and showed how Uchihas abused Izanagi. They use Izanagi with the intent of getting the dominion over others. So another guy, with his own Izanagi, would kill the first one who used Izanagi, then another one then another one until there's a cycle of killing between selfish Uchihas. Izanami is used to prevent them from abusin it. How? The first "picture" would be the moment before Izanagi is triggered. Therefore, the Izanagi user would be stuck in a loop from the moment he still has not used Izanagi to the moment he used it. It's also the first time a kunoichi is shown to use a sharingan. Due to the flashback, it's confirmed that Obito either lied or just claimed that people with both power of Uchiha and Senju can use the power of Izanagi.

Naori, the one who used Izanami to save her

comrade from Izanagi abuse.

Raiga Arc (Naruto, not Shippuden)- This is the first time Kiba (swords) were shown. It was used by a former Swordsman of the Mist named Raiga. He was later shown in the manga, at least presumably. This is due to the wielder of kiba in the picture not being ringo but somebody else who has the same thick lips and eyebrows as Raiga.

Raiga, the wielder of Kiba before Ringo.

Kakashi ANBU Arc: The name of the arc says it's all about Kakashi, but in fact it's not. He's just the center of the story but this Arc showed answered a lot of things to be clarified and shown:

Kakashi's life after Rin's death: It showed that he became very depressed about it. Kinda plain right, but this would link out to other events:
How Minato was promoted to being the Fourth Hokage: It was shown that he became the Hokage before Kushina was pregnant (or at least not yet confirmed through medical check up).

How Kakashi became part of Konoha's ANBU: Minato put him into the ANBU to erase the "darkness" and depression in him. It would give him (Minato) more time to be with his student.
What happened after Kushina and Minato's death: It was shown that Kushina didn't die, at least immediately, after Minato used his Eight Trigram Seal to seal the all of Yang Kyuubi, His and Kushina's chakra inside Naruto. Therefore Kushina did have a very strong life force for staying alive for few minutes after all of her chakra had been drained. She entrusted Naruto to Hiruzen after Minato's barrier was dispelled by his death, therefore allowing Hiruzen to get close to them.

Kushina entrusting Naruto to Hiruzen

Hiruzen is carrying Naruto

Yamato's roots: It was shown that Yamato never knew his true identity. He never remembered anything other than seeing his fellow experiment specimen while being inside one of Orochimaru's capsule.

Kinoe inside lab capsule with his only known

"friend" in his memories

Therefore his last known name was his Root ANBU (The Foundation) codename Kinoe. After doing several missions under Danzo, he met Iburi clan. One of them is Yukimi who calls Kinoe "Tenzo".

Yukimi

Apparently Tenzo was also Orochimaru's test subject and looks a lot like Tenzo. It was never made clear if Yamato is actually Tenzo. It was hinted that he's not Tenzo, but no direct answer was given because of the fact that Yamato never learned his roots. Yukimi was shown to be 100% sure that Yamato was indeed Tenzo, so it can be a thing.

Tenzo befor being taken by Orochimaru

Eventually Kakashi met Kinoe due to conflicting missions: Kinoe was to ensure Orochimaru's safe arrival, while Kakashi was ordered to kill/subdue Orochimaru.

Later in the filler, it was shown that Danzo grew suspicious of Kakashi and ordered Kinoe to get his Sharingan. Kakashi was ordered for another mission at this time: go to one of Orochimaru's abandoned lab. Well, Kinoe can't betray his friend and sent Danzo a fake Sharingan made of wood. This angered Danzo and ordered to put a seal to Kinoe so he'd follow Danzo without question. This was stopped by Kakashi, who was ordered by Hiruzen to go to Danzo's hideout to summon him to the Hokage's quarters. Hiruzen eventually asked Danzo personally to take Kinoe to his own ANBU and therefore relinquishing Kinoe of his duties to the Root.

Once in the Konoha's ANBU, he was called by Kakashi "Tenzo" though Kinoe said that he's not Tenzo. Anyway he accepted the name and people in the ANBU calls him by that name, recognizing that Kinoe's real name is Tenzo. On Tsunade's term, he was then assigned to be a replacement to Kakashi and was given the codename Yamato

Orochimaru's snake form; It seems like Orochimaru first created his monster snake form as a separate entity before finally merging himself to it. Kakashi and Yamato fought a similar monster inside one of his abandoned laboratories.

How did Kakashi became an ANBU and a Root ANBU at the same time: He was never a full Root ANBU. He just received missions from Danzo when he was convinced by him. Eventually Kakashi decided to receive missions only from Hiruzen.
Proof that Danzo stayed his contact with Orochimaru: Before Orochimaru was ambushed by the ANBU, it was shown that Danzo was talking to him about certain experiment.
Danzo's left eye: Before it was Shisui's eye, it was a sharingan also capable of doing a powerful genjutsu.

Itachi's ANBU life: He was assigned to ANBU due to his skills. Eventually he was convinced by Itachi to work for the Root... which is a huge mistake. Eventually he became a double spy and took orders from his father, Hiruzen and Danzo. He was never agreeing with the coup.
True history behind the massacre: Hiruzen doesn't want to have a huge conflict to happen. Instead he wanted to talk to Uchihas. He even went as far as to give Uchihas the duty of patrolling the village to ensure the security. This is something the idiot Danzo doesn't want. Plain and simple, he doesn't the Uchiha to gain power, he wants the power for himself. Therefore he proposes to fight the Uchiha not head on (he knows they'll lose if they fight Uchiha head on due to their sharingan). However Hiruzen managed to equalize the conflict by evading the conflict and proposing to just make 24-hour surveillance at the Uchiha Village. At the last meeting of the Uchihas before the coup would commence, Shisui proposed to take the matter into his own hands. Like Itachi, he doesn't agree with the coup too. So he went to the Hokage with the permission from high Uchiha command. However he doesn't side with the Uchihas, he wants to stop the coup. However he also tells the Konoha government on what the Uchiha wants. Hiruzen acknowledged it and orders Shisui to take matters into his own hands.

Shisui plans to talk to the Uchihas and use Kotoamatsukami if ever the need arises. This would make the Uchihas stop the coup without them knowing that they are being controlled (he'd control Itachi's father, as he is the leader of the Uchiha). Danzo, being selfish that he is, became envious of Shisui's power and wants to use it to "take matters into his own hands". He then confronts Shisui to take his eye...
How Danzo took Shisui's eye: He used Izanagi to beat Shisui and take his eyes. He was first incapacitated by Shisui through strong genjutsu, Izanagi would beat anything as long as it is active. Shisui, being the fastest known Uchiha, escaped. This was the time when he and Itachi met on the cliff they used to hangout. He then told Itachi that Danzo took his eye and gave Itachi his another eye because, according to him, Itachi is the only one he can trust. He then jumped down the cliff (assumably to his death) telling Itachi to not follow him...

Izanagi-Danzo beating up Shisui

Shisui's eye taken

Danzo ordered the Massacre: Desperate Danzo then ordered Itachi to kill the whole Uchiha. He persuaded Itachi by using Sasuke. He said that there's a chance that Konoha will win against Uchihas, therefore the Uchiha will be extinct together with is brother. Itachi said that he'll do the murder, but Danzo must leave Sasuke alone. Danzo agrees, but on a suspicious manner. Eventually he meets Obito who seems to trying to do something bad. Itachi then said that he'd help Obito (who he thought as Madara, though he has an idea that it's someone else) kill the whole clan, but leave Sasuke alone.

Itachi did not single-handedly killed the whole clan: He and Obito did it.

Obito helping in the massacre

The Massacre was never known by the Konoha Government. It was a decision from Danzo without approval from the Hokage or the Elder Advisors: When Hiruzen learned about it, he went to the Uchiha village to see the incident. He was shocked. They only saw one survivor: Sasuke. He then furiously deprived Danzo of his own authority. Therefore...

Aftermath of the murder

All of Danzo's missions without the Hokage's approval are illegal from then on: Being deprived of his own authority, the Root missions are all illegal except when approved by the Hokage.
Itachi regretted the decision, the reason he can still get inside Konoha and why he is coming back to Konoha: Itachi went to Hiruzen to ask him for Sasuke's safety. He said that he trusts Hiruzen to take care of Sasuke from Danzo. Hiruzen did not doubt, he agreed without question. He asked Itachi of his future plans. Itachi said that he knew of a man who's a part of Akatsuki and he wants to keep an eye on him in case he breaks the agreement. The man was Obito and the agreement was Sasuke's safety. Hiruzen said that he would not change the password to Konoha's barrier so Itachi can enter Konoha again whenever he misses Sasuke or just want to check on his status. Itachi then regretted the decision. He somehow said that the Uchiha should have known about Hiruzen's intent from the start and stopped the incident from the start:
How did Kakashi become a teacher: Hiruzen noticed that Kakashi's darkness still haven't vanished after all that happened. He decided to put him to become a Jonin leader (teacher of Genins). He thought that teachers learn from students too and the appreciation and results of his handiwork would be a great way to take his darkness away. He was always strict, never passing anyone as long as they don't do even one of shinobi' main principles: Teamwork, never leave your allies alone and rules. Eventually the students thanked him... well actually....