The fact that Urabe and Tsubaki never kiss in the entire series, making it feel like a "Shaggy Dog" Story for many fans.
Genius Bonus: The covers of the books contain rather large portions of the English language versions of various literary classics — the original Frankenstein for Volume 1, for example.
Les Yay: Urabe and Oka, exchanging saliva whenever they have lunch (not like that! Granted, it's kinda kinky all on its own...).
Oka peeking down Urabe's top.
Not to mention her camera habit — she takes pictures of Urabe any time she gets the chance (including a nude shot from that confrontation with Hayakawa). In the OVA, she takes one group shot before the double date, but then starts snapping artistic solo pics of Urabe, leaving the guys wondering what the hell is going on.
Magnificent Bastard: Akira's ex-crush, Hayakawa. To the point that she's openly taunting Urabe, faking being in an abusive relationship, and suckering Akira into faking a date with her — at the same time she's getting Urabe to show up to "accidentally" catch them so he'll take the fall for it. Turns out Urabe was one step ahead of her...
One True Threesome: Latest chapters a slightly teasing with Tsubaki/Urabe/Suwano. Subverted in chapter 86.
Another possible is Tsubaki/Urabe/Momoka which is nearly Twin Threesome Fantasy thanks to Urabe's and Momoka's very similar appearance. Just too bad, that Tsubaki actually doesn't know about their "twin switch".
Squick: Sucking saliva off someone's fingers may not be the most romantic thing for most people.
In the anime it's drawn a lot thicker and gooier than it looks in the manga, with a slight yellow tint.
Tsubaki, multiple times. Part of it is due to his na´vetÚ, part because he has had the Idiot Ball surgically implanted and partly because his girlfriend is, quite honestly, really weird.
There's also Ueno, who still has no idea that Tsubaki and Urabe are dating — they may not say it out loud, but their reactions to each other are very unsubtle.
Urabe gets the Idiot Ball in the base-breaking Idol arc when kidnapped and forced to pretend she's the said idol. She just goes along instead of grabbing scissors and escaping ASAP. Coupled with Tsubaki's inability to realize the idol is pretending to be Urabe, despite her doing a poor job at it, and despite that "Urabe" has smaller breasts than usual, which he's already groped and seen exposed...