These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ascended Fanon: Fan speculation (NSFW) led to the girl sitting next to Yui in class being referred to as Himeko Tachibana. The name itself came from an official list of classmembers, but no specific character was attached to the name, and while the girl in question did have a line, she was only credited as "Classmate". The character profiles in the Season 2 DVDs confirm that she's Himeko.
Awesomeness Withdrawal: Common reaction for people who love the series and watched it from beginning to end... it's not easy to let go of a happy childhood after being allowed to go back to it one more time.
Badass Adorable: Mugi could be considered this, especially given how strong she apparently is and the fact that she knocked Akira head over heels with a "light blow."
Just to prove her strength, while keeping in mind that she has a keyboard which she regularly carries with her to and from school, take a look at this.
When checking out the other clubs, the girls visit the Occult Club, which consists of a big, blinking UFO sitting in the middle of the club room. They consequently close the door and never talk about it again.
Covered Up: This doesn't apply to any Japanese person since it's a classic pop standard and children's song, but face it—the only reason you know about "Tsubasa o Kudasai" is either because of this show, or Evangelion: 2.0.
The dub replaces it with "Love Me Tender".note This is still in keeping with Covered Up, since the tune of "Love Me Tender" was based on a Civil War-era song called "Aura Lea", but the Elvis version is much more famous.
Crowning Music of Awesome: The theme song "Cagayake GIRLS!" shot up to the top of the charts right after the pilot episode aired.
"Fuwa Fuwa Time" as well.
Pretty much the entire soundtrack qualifies, including most of the image songs.
Particular mention goes out to the cast not only singing, but also actually playing the instruments of their respective character for two songs on the second Radion Special CD.
The second season opening theme, "GO! GO! MANIAC!", goes beyond that and manages to become the first song credited to fictional characters to reach the top of the weekly chart, while ending theme, "Listen!", also reached second place. Hokago Tea Time also manages to become the third female act to simultaneously place at the first and second spots of the weekly chart.
Adding to the lineup is the second season's 2nd OP/ED set
Episode 24. Not only did they write & play a song ("Tenshi ni Fureta yo!") just for Azusa, but all the graduating members get a shot at the vocals this time, and the song is so obviously about the graduation. Just so you know, this doubles as a CMOH. So hard.
Even after the series ended, Mio's poem about caramel sauce that everybody hated when she read it to her fanclub back in season 2 episode 7 makes a triumphant return as "Tokimeki Sugar", one of the best songs on the Houkago Tea Time II album.
There's Ichigo in Episode 18 who, despite appearing in only a few shots in the season, is gaining fan art.
Averted with Satoshi, the one boy in the cast, who appears in relatively little fanart. If he is there, he's either with Ritsu or Mio. Being male may have something to do with it...
K-ON! is one of the only, if not the only, anime whose entire 38-student HS homeroom class has been fleshed out, with names and full character designs. Some of the classmates: right of Yui is of course Tachibana Himeko, and in front of Mio is Wakaouji Ichigo, but behind Mio is Nakajima Nobuyo, to the right of Mio is Miyamoto Akiyo, to the left of Mio is Takahashi Fuuko. Behind Ritsu is Endou Michiko, to her right is Wajima Maki, to her left Chikada Haruko. In front of Mugi is Sano Keiko, behind is Kinoshita Shizuka, to the right is Sunahara Yoshimi, to the left is Taki Eri. ALL of them have names, designs, and for several, even several lines of dialogue.
Unfortunately the series still manage to fall into the Only Six Faces trope. Take away the hair and any glasses, everyone looks identical (with the exception of Mugi, mainly due to their eyebrows).
Fandom Rivalry - Whatever you do, do NOT mention both K-On and Haruhi Suzumiya in one sentence at websites where fans of those shows are. (Since they're mostly infested with trolls.) The odd thing is: While the creators for the manga/light novels are different, they were both made as animes by the same company, Kyoto Animation. The rivalry stems mainly from the fact that both K-On! and the second season of Haruhi were produced around the same time, and both (supposedly) suffered for it in terms of quality storytelling. Plus, there are moments in Haruhi where the characters resembled K-On!'s characters.
Also, fans of Mecha absolutely loatheK-On!. They blame it for spurring on endless Otaku-panderingimitators in a Moe boom that saturated the market and has made quality mecha far rarer than it used to be.
Fan Hater: Some rabid haters tend to accuse anyone who even so much as likeK-On! of being shallow, stupid Moe obsessed otaku who don't want substance in their anime.
Sadly, there isn't much to alleviate those claims if the Facebook fanpage is any indication. 60% of the comments on any given picture will be "kawaii" and many will not fail to add the "-chan" honorific when referring to any of the girls. Such generalizing is still unfair to the rest of us though.
Such insults shouldn't be raised at all. There's nothing wrong with liking Moe.
Fan-hating led to So Ra No Wo To being blind bashed as Military K-On! for having similar artwork, featuring a five girl tank crew, and music playing a strong role in the show.
Fan-Preferred Couple: Ritsu/Mio, for certain. Yui/Azusa also count to a lesser extent, even though recent chapters of the manga seem to be Ship Teasing Yui/Akira and Ui/Azusa. And then there's Sawako/Mugi. The Fan Preferred Couple among the background characters is Akane/Eri.
I Am Not Shazam: First time watchers would be forgiven to think the girl's band name is K-On. Its actually Houkago Tea Time.
I Knew It: Because the final bonus episode had the girls getting their passport pictures taken, fans predicted that the film would be about the girls going on a trip to another country. ....guess what!
Memetic Sex Goddess: In the Anime wives poll, Mio ranked number 1# twice, and Yui and Azusa made it into the top ten in 2010 and 2012 most wanted Valentine they made it into the top three.
Moe: All five girls. Yui's sister Ui qualifies too. Mio, however, deserves special mention, her Moe-ness being over-the-top even by the standards of this show, spawning a Running Gag. And when Azusa arrives, the show causes a huge moe explosion that obliterates everything within a 100-mile radius.
Never Live It Down: Nobody will ever let Mio forget the time she tripped on the stage during the light music club's first festival performance.
Nightmare Fuel: During the sequence in the first episode of the anime, when Yui is walking down a hall with the "paranormal" clubs, a very dark and sinister-looking girl (looking similar to Enma Ai from Hell Girl) pokes her head out of a door on the left.
In the second season, they briefly look inside the Occult Club and see a UFO inside. Not really that spooky, but remembering the aforementioned girl from the paranormal clubs, one wonders what the deuce is going on in those rooms...
And in episode 21 season 2, Ui can be briefly seen wearing her sister's hairpins when greeting Azusa, paving the way for the very next scene.
In one of the final episodes the band decides to record a cassette of their songs for posterity. The mix is included with the second album, and it includes everything that was shown to be recorded in the episode, including Ritsu talking about "rock spirit", Azusa yelling, and Tsumugi sneezing.
Rule 63: You better believe it! Someone actually managed to gender-flip the songs themselves!
Suspiciously Similar Song - The tracks "Virtual Love" and "Pinch Daisuki!" sound suspiciously similar to "Das Model" by Kraftwerk and "Popcorn" by Hot Butter respectively. Not that that's a bad thing, mind.
Ritsu and Sawa-chan-sensei's diabetic seizure in reaction to Mio's sincere but almost Sickeningly Sweet lyrics of "Fuwa-Fuwa Time."
They Wasted a Perfectly Good Plot: A common criticism of the show is that it does not focus enough on progress of the band and their music, as opposed to the more common slice of life elements. Only 4 songs are introduced in the body of the show, and the band doesn't get popular beyond their own school.
The Woobie: You really can't help but feel bad for Mio, what with her extreme shyness, her painfully adorable Cry Cute moments, and the fact that Sawako and Ritsu often make her do embarrassing things. The poor girl really needs a hug sometimes.