Karasawa, the Literature Girl, Ringo-chan, Habara/Archdemon and NAGO. Several popularity polls in, and Karasawa and Habara in particular seem to have permanently occupied some of the top places, coming in (as of the March 2012 poll◊) in 1st and 2nd place, respectively.
Given Tadakuni has been seriously Out of Focus after the first volume of manga and the fifth episode of anime, his position in the polls—far beyond Hidenori and Yoshitake—also makes him one.
Friendly Fandoms: The relationship between this show's fandom and Nichijou's fandom, both due to them being similar in name and due to them sharing a genre. Both will argue about which show they think is better, but the arguments never get heated, and both fandoms seem to respect each other and their respective shows. If someone has only seen one show and not the other, fans will often urge them to watch the other as well.
In High School Boys and Ghost Stories 2, Tadakuni tells a story about seeing what's under Karasawa's hat and only says it's hilarious, setting up a chase scene after Karasawa to take his hat, and Tadakuni saying in the background he made it up. Fast-forward to a few chapters/episodes later where Karasawa shows he does have something under his hat, namely, a scar from his confrontation against the Archdemon.
In High School Boys and Morning Journey, if Tadakuni, Hidenori and Yoshitake are role playing any of Square Enix games (particularly the Dragon Quest,) then come Final Fantasy Type-0 which Eight, King and Jack are voiced by the former.
Ho Yay: Hidenori and Yoshitake has been prominent, due to their Mistaken for Gay moment, and then there is High School Boys and Annoyances...
During " High School Boys and Dropkicks", Hidenori encounters a girl, Emi who kicks him because mistook him for a friend she knew, and the aforementioned friend (Kiyohiro) who befriends Hidenori instantly after Hidenori tells him to give her payback for all the times she's kicked him. They then proceed to spend nearly the entire summer together. There's even a Lampshade Hanging from Emi herself and the opening credits of the anime episode the skit was featured in, questioning if this was "boy meets boy".note The show said yes, but it wasn't that kind of show.
And of course, there was the infamous "High School Boys and Skirts" segment, where both Yoshitake and Hidenori admit that Tadakuni looks good in a skirt and proceed to have him put in his sister's clothing.
Shipping: If after the author's comment that the series is strictly a comedy with No Hugging and No Kissing, you'd expect that something like this wouldn't happen, you thought wrong. Indeed, there are several fan-favorites just from the anime alone, including but not limited to:
Hilariously all the pairings listed above (save for Karasawa) were featured in the episode 12 skit High School Boys and ... only to be sunk quicker than the Titanic due to the skit itself being All Just a Dream.
As homosexual ships are concerned:
All possible seme/uke combinations of the main trio are being shipped. Of course, since Tadakuni is an Attractive Bent-Gender, he's more likely to be the uke.
Motoharu and Karasawa
Signature Scene: More like "Signature Skit", but when most people think back to this series, they think of "Highschool Boys and Skirts".
Unpopular Popular Character: The trio from High School Girls Are Funky desperately try to become appealing characters to branch off into their own show, only to inevitably fail in the process. Outside of the series, they are extremely popular characters.
Vanilla Protagonist: Tadakuni is a Generic Guy with no backstory and has fewer relationships with other characters than his two friends Hidenori and Yoshitake. Tadakuni's suffers from being Out of Focus, with no presence in some episodes and many chapters. His status is often lampshaded. He's Out of Focus after the first volume and episode 5, making him somewhat like an Ensemble Dark Horse.