Author Appeal: clothes. Hayashiya Shizuru plainly enjoyed playing dress up the characters: even Saeki sports a different hairstyle practically every episode, and in Zlay-san's case, it verges on Costume Porn. Sera, the photographer, is the only character who keeps more or less the same looks throughout the series, which kind of makes her stand out from the rest of the cast.
Closet Key: Ran is this to Julia, who didn't even know what it was like to feel attracted to another person (let alone another girl).
Concert Kiss: Subverted -Ran and Julia kiss for the first time on stage at the Zlay concert, but there is nothing romantic about it as it's just part of the performance; their real "first kiss" comes a few pages later, while they watch the concert from the backstage. Arguably counts as an Exploited Trope too, as it's heavily implied that Genre Savvy Ryou had set everything up in order to bring it about.
In Vino Veritas: One glass is enough to transform the shy, insecure and -up to that point- mostly straight Ran intothis
Intertwined Fingers: Julia and Ran doing it in chapter 15 symbolizes the next stage of their relationship.
The Jail Bait Wait: Kaoru is careful to point out that she has no intention of "crossing the line" with Haruna as long as the girl is under the legal age of consent. Teasing her to no end is a different matter, and apparently covering each others' bodies with kiss-marks is allowed, causing an exasperated Saeki to ask what line is going to be left to cross at this rate...
Lost in Translation: According to Word of God, much of the humor is based on puns or references to Japanese popular culture and media. Western readers who only know the manga from Scanlation are probably missing out on a lot...
Not What It Looks Like: Several, though one in particular was during New Years when Saeki, pissed off at Kaoru's flirtations, ties her up and threatens to throw her out with the trash. Cue Julia and Ran walking in at that very moment and add the fact that both know Kaoru has a thing for Saeki...
Punch ClockAntagonist: Saeki is constantly blocking Julia's advances towards Ran. However, at first she's under the assumption that Julia is just going through a phase, and it doesn't even cross her mind that Ran could reciprocate her feelings; later, she's just doing her job, trying to avoid a scandal that might destroy the two girls' careers; in the end, she's more than happy to quit playing chaperone for them.
Schoolgirl Lesbians: invoked by Saeki, who keeps insisting that Julia must "grow up" and give up on Ran. Ultimately averted.
Screw the Rules, I Have Money!: Averted! Even though Shizuka is a famous actress with lots of power and influence, it doesn't stop Julia from putting her in 42 joint locks.
Seme / Uke: Invoked by Word of God (in the notes at the end of the first volume), but totally avoided in the manga: while Julia simply can't work up the courage to make her move, Ran struggles through several chapters to control herself and not pounce on Julia. In the end, Julia confesses first, yet she still thinks she's the "passive partner" because Ran takes the initiative for the kiss; and the dynamic leading to that moment has nothing of the Seme/Uke in it.
Rena, on the other hand, goes full Seme once she gets over her Tsundere phase.
Unresolved Sexual Tension: What the plot revolves around. Chiefly between Julia and Ran, but also between Rena-chan and Mutsuki. Also, played with between Kaoru and Saeki: since practically the whole cast are gay, it is heavily implied that Saeki might actually be a closet lesbian and that Kaoru's lust for her might not be totally unrequited. This turns out to be a red herring, though.
Unwanted Harem: Things get pretty hot around Saeki in chapter 4. Sadly, it only lasts for that one chapter.