Animate Inanimate Object: Sex toys in this series are moderately alive and remember everything that's done to them. A fairy occasionally shows up to bring them to life, but it tends to turn out badly, since they know nothing other than how to give sexual pleasure. (The longest-running arc with one featured Shiunji's former blowup doll Ishida's dug-up corpse, who tried to take revenge upon him and found that he was so perverted he would have been happy to be cut into bits. Then she got torn to pieces by dogs.) It's bizarrely tragic, albeit funny and cringe-inducing as well.
Art Evolution: Early comics were in black-and-white, and less detailed.
Ashes to Crashes: The nymphomaniac mother kills her son's girlfriend via sexual exhaustion. She attends the funeral. The urn her ashes are in is vaguely phallic. She tries using it as a dildo, and it breaks ...
Ass Shove: Often the least offensive joke in a strip.
Brother-Sister Incest: Mike has masturbated to his sister several times - and masturbation is the only form of sexuality to him. In one strip, he needs to answer the phone while they're both watching porn together, and asks her to reach over and "take over for a while". Later strips with the two of them continue tightrope-walking along the line without quite crossing it.
Even extends to seemingly unconnected Running Gags - if Word of God (through the site's navigation) is to be believed, the strips with the punchline at the bottom of this page all feature the same woman.
Creator Breakdown: Literally—the writer suffered an injury to his drawing hand. Also figuratively, as he's struggled with ongoing depression issues - though these sometimes have served more as a source of inspiration and sometimes as an obstacle to it.
Flanderization: The Swearing Friend. He used to be able to have conversations, and sometimes made it through an entire sentence without swearing. By the end of the strip's run, he did nothing but swear so much he was borderline incomprehensible.
Not What It Looks Like: No matter how often he finds his wife in bed with his best friend, the Cuckolded Husband believes this immediately, and runs with their lame excuse until they wish they'd never said it.
In #31, a guy caught cheating on his wife tries this excuse. Unsurprisingly, it doesn't work.
Primal Scene: Kenta walks in on his parents having sex (which includes bondage and roleplaying as DC Comics characters).
Prison Rape: One character admits that he has recently come out as gay, he would actually enjoy being the passive participant in the experience, and the assertion of dominance involved would not be psychologically traumatizing or break his spirit. This actually prevents the rape, because the rapists are mostly interested in making him suffer.
Quote Mine: A non-verbal version with rather unfortunate results in strip #219, which takes place at a book signing.
"JERK OFF NOW!" -> "IN MY FACE"
Madame X: What a tragic pairing of titles. You ought to read the guest list before agreeing to these things.
Chie: Shut up and go get some club soda to pour on me. I'm stuck to the chair.
The Rant: Parodied in thisSunday Morning Coffee guest strip.
Or, for that matter, all of them. Word of God is that "everyone is locked into their own perversions", which might be part of why the comic has apparently stalled and died.
Anyone accomplishing anything would kill the joke (according to Clay), so no, Chie would never get to have normal sex, Kenta's mom will never bed her son, Touru will never get Madame X to have sex with him, Mike's roommate will never learn to live with Mike's habit or move out, etc.
Suicide as Comedy: There's a running gag of people being unwilling to go through with it when they realize Shiuji will rape their corpses. Also, one girl tries to fake suicidal tendencies so Shiuji will be interested in her, then realizes she has no easy way out of the situation.
Played straight as early as #8 (NSFW), where after sex, the woman reveals that she's not on the pill. When the man freaks out, she calms him down, saying that she can't get pregnant... because she used to be a man.
Viewers Are Geniuses: The writer would often throw in this or that element of Japanese culture and expect everyone to not only be familiar with it, but to recognize it on sight with little buildup.
Webcomic Time: The resolution of "Shiuji and the Suicide Girl", set during a single day, started in 2004 and ended in 2011.
Wrong Name Outburst: In strip #169, Rumiko and Natsuki both call out Kenta's name while having sex with each other.
Yet Another Christmas Carol: Mike goes through one of these, focusing on the past and present of his history of treating others as sex objects rather than human beings, with the message that he will die alone and unloved. He completely fails to get it.