Jet Pack: All rocket-based means of transportation in the comic have the same red-and-yellow flame pattern. John has a standard issue jetpack, Bro has a rocket board, and Vriska has Rocket Boots. Vriska handed out the code for her boots to her teammates, allowing Terezi to make a pair of wings, and Tavros a Flying Car. Gamzee has the Unireal Air: A unicycle with rockets attached to it. It's about as useless as it sounds.
Played straighter with Bro: One katana slice is all it takes for him to cleave a meteor headed straight for his and Dave's apartment perfectly in half.
Outright averted with Dave; Dave's strongest swords are based on European swords.
Exaggerated and parodied with Dirk Strider.
You retrieve your UNBREAKABLE KATANA. A real hard sword for a real hard dude. It was said to be forged by an ancient Otaku Master over the heat of a roaring manga fire. It was cooled in an enchanted spring where virgin horses nicker and bathe, and was said could be used only by one whose pointy anime shades were deemed sweet enough, and whose hair existed in a perpetually sculpted state of looking completely fucking awesome. All of those things were said by you.
Kid from the Future: Jade's penpal, J, claims to be her grandson. This turns out to be mostly true in Act 6 (the hitch is there are two different Jades, and that genetically J (Jake) is Jade's father.)
Killed Off for Real: So far none of the main cast, but several minor characters have been killed completely, beyond even Dreambubbles. The introduction of Dreambubbles is all that keeps some segments of the cast in regular panel time.
It's actually really hard to get absolutely, finally Killed Off for Real in the Homestuck universe — there's only two known ways. You either have to die normally (usually at least twice, on account of dream selves), then get your ghost killed again, personally, by the Big Bad, who is the only one who can kill ghosts permanently; or you have to be alive and not a ghost inside an exploding universe when it gets Scratched. And even those two things don't keep you from being brought back by Weird Time Shit or by having an identical duplicate created by a Scratch, new session or the like.
Kiss of Life: If a player dies while they still have a viable dream self, a kiss to the dead body will cause the dream self to become their new body. This comes up in recurring fashion, often with several kisses coordinated at once.
Kissing Discretion Shot: In [S]: Synchronize, Dirk kisses Roxy to wake up her sleepwalking dreamself, but just before he does, the kiss is covered by a speech bubble blurted out by Squarewave.
Klingon Promotion: In troll society heiresses to the throne (like Feferi) were expected to challenge the Empress for the throne once they reached maturity.
Kudzu Plot: Very much so! And Andrew is extremely good at keeping plot holes from happening. note There have been a few small goofs in art and timelogs, but aside from that he's been pretty much perfect about plot holes.
Language Equals Thought: "It should be noted that in troll language, the word for friend is exactly the same as the word for enemy."
Late-Arrival Spoiler: Both averted and played straight. The Trolls becoming major characters is a major spoiler, but they appear as far back as Act 3, and were heavily hinted to be fairly important. On the other hand, the mere existence of the B2 Session is a huge spoiler for Act 5.
Also, many pictures can be massive spoilers. In particular, any picture of Jack Noir in his final form contains massive spoilers for the entire first half of the story, revealing not only Jack's ascendance to Big Bad but also all four of the kid's pre-entry prototypings (especially Bec.) Likewise, pictures of anyone in God Tier clothing can be a spoiler, though there's enough fanon pictures of people who never ascended (and even canon pictures of alternative dream selves that ascended) that it's not always telling. For that matter, Jade's name is a spoiler — everything about her was kept completely secret for the first few acts.
Here, when Jake asks Dirk's Auto-Responder if it deliberately arranged things so that Jake would have to kiss Dirk's severed head to save everyone. The Auto-Responder replies that it would have to be moderately sociopathic to deliberately arrange something so unsettling, not to mention a genius.
Leitmotif: Most characters have not only a song (or several!) associated with them, but also an instrument.
John has Showtime and Doctor as his themes. Showtime was his theme for much of Acts 1-4, whereupon Doctor more or less superseded it. Doctor has since been bootstrapped into being a theme for God Tiers, so the current "main" theme for John is nebulous. John is also associated with piano music in general, and organs to a lesser extent.
Dave has Beatdown, Atomyk Ebonpyre, and Sunslammer as his themes, the last serving primarily as a Recurring Riff. Dave is also primarily associated with synths (though not to the extent of the Trolls), turntables, and drum kits, as well as guitars to a degree.
PM does not have a specific theme, but is heavily associated with Sburban Junglenote Her appearance in Cascade coincides with the only appearance of the riff and the only song identified as being solely about her on the Wanderers album is a remix of it.
The other Exiles have their own themes though none have been used in the comic itself so far:
Meenah is associated with dubstep style wubs and her theme is Fuschia Ruler.
Kankri is associated with church music and his theme is Iron Infidel.
Latula is associated with harpsicords and orchestral music like Terezi, but with cheesy Gamer Girl beats and raps. Her theme is Game Glr (A remix of Teal Seer and Game Grl), unreleased as of yet.
Porrim is associated with etheral ambient synths like Kanaya, only taken in a more cheerful and melodic direction. Her theme is Darling Dolorosa, unreleased as of yet.
Time in general is associated with a clock ticking, whether that's with the Heroes of Time (Dave, Aradia, Damara, and Caliborn), or the Felt.
Letters 2 Numbers: Many of the trolls do this as part of their typing quirks, usually in a way that fits into some sort of Numerological Motif. Terezi, though, deserves special mention, since with as many as three letters constantly numberised, turquoise color and all-caps H3R T3XT C4N B3 N1GH UNR34D4BL3.
Light and Mirrors Puzzle: A key aspect of the interactive segments at the beginning of Act 6 Act 3, wherein Jane explores the Land of Crypts and Helium. Four colored lanterns at a shrine emit beams of light that reflect off mirrors in different directions depending on the color of the light. (Must be a wavelength thing.) There are TONS of mirrors around to reflect the light, and several switches that can only be flipped when one of the beams is shining on the switch. By having Lil' Sebastian rotate the lanterns, Jane can change which beams shine in which directions, and therefore which way the mirrors reflect them, and therefore which switches are flippable.
Lighter and Softer: Though Act 6 still has dark tones throughout, it is still more lighthearted than both parts of Act 5 were.
Like Reality Unless Noted: If you copy down the GPS coordinates that appear in one of the videos, and type them into google maps, you'll find real-life locations corresponding to John's neighborhood and Rose's house.
The coordinates for Jade's island correspond to an area of the Pacific Ocean with nothing... except a small underwater rise that looks as if it could have been an island with just a little more volcanic activity...
Limited Social Circle: John, Rose, Dave, and Jade aren't shown talking to any other kids their age. Except for the trolls, they literally have no one else on their Chum List. It is also not known if they even attend school. The only exception is Jade's penpal who claims to be her own grandson.
The post-Scratch kids aren't any different, although to be fair Roxy and Dirk live After the End, so they don't have much choice.
There are four human characters, and twelve trolls, each of whom has an ancestor who is also an important character. And each of these characters (both the ancestors and the descendents) has both a prescratch and a postscratch version. Oh, and they all have sprites who are also distinct characters (though sometimes composed out of other characters), and most of them also have an associated Exile who is also a distinct character, plus a Denizen who serves as a major boss and sometimes a mentor. Plus each session has its own copy of the core 'NPC' cast, plus the Big Bad also has fourteen unique servants. Also, any of these characters can and do leave alternate-timeline ghosts which can become their own characters. And even then, there are a few other random characters that don't fit in anywhere else, like the firefly, Serenity. And did we mention that all the important kids and trolls have at least two names, some of which aren't revealed until very late in the story?
One example of how many characters there are is in this page, which shows a smackdown with all their ghosts (including many timeshifted duplicates, but excluding most living or non-player characters.) Another example is here.
Lord British Postulate: It seems Eridan wasted a lot of time killing the NPC-equivalents on his planet, and they were "fast and angry as shit".
KARKAT: THEY DIDN'T EVEN GIVE YOU ANY GRIST, YOU IDIOT. THAT WAS YOUR FIRST CLUE.
Also Lord English, an indestructible demon that has to be defeated by exploiting Stable Time Loops and various glitches. Note that this was a retroactive reference.
Long Title: Apparently all of the troll's movies have these, due to having run out of short ones in their longer-running society.
Karkat makes fun of this when he creates a Shipping Grid for the prescratch human players; since they are the only four humans left in the entire world, and they consist of two pairs of male-female siblings, there is exactly one and only one way to pair them up at that point in the story.
WV and AR are both infatuated with PM. PM has not demonstrated a real preference for (or interest in) either so far. Smashed with AR's death but revived with Noir's crush on PM.
Eridan has (or perhaps "had") a thing for Feferi, who is in a relationship with Sollux. Eridan is also on the lookout for a Kismesis and sees Sollux as a possible candidate.
Nepeta has an unrequited crush on Karkat, who has (had?) a thing with Terezi.
Kanaya loves Vriska who is attracted to Tavros who's afraid of Vriska.
Karkat and Terezi have some nebulous relationship...but Terezi seems to be cultivating something with Dave. Then again, Dave calls his talks with Terezi an "interspecies partnership in incredibly shitty cartooning".
It's later revealed that between intermissions Dave did date Terezi offscreen, but when she "secretly" became Gamzee's kismesis, Dave quietly stopped seeing her, mostly (he says) because he can't wrap his head around the idea of "black romance," and would have no idea how to behave because he can't tell whether something is "fucked up but normal for trolls" or "fucked up because it's just plain fucked up."
Lower Deck Episode: Part 1 of Act 5 is one for the Trolls, as it focuses on their session of Sburb without input from the Kids, and an "intermission" between acts 3 and 4 focuses on the Midnight Crew, with no indication that they're even connected to the main story until the very end, when Spades contacts Karkat (who the audience could not even recognize in person yet) in the same way WV did John.
Inverted later; the intermissions in Act 6 are the only times John and Jade have shown up as of yet, while the cast on the Troll's meteor appears in the normal acts as well.