Steve almost always says "HI GUYS!" when he first appears in an episode.
The Inaudible Thwomp's subtitles will usually change into "???"
Paul Hammerbro throws hammers when angered.
Kamek constantly calls Jeff and Hal (mostly Hal) gay.
Brawl of the Objects had a character named Hot Dog, who doesn't like catching small, gray boxes because he keeps missing them every time he is safe from elimination. He finally managed to catch a box during the first mini-challenge in "How it All Ended," when Pizza told him to focus.
Boomstick regularly bringing up either his Disappeared Dad or his horrifying ex-wife. The former gag end up as the center of its own arc throughout season 8, and would eventually be put to rest with "Macho Man VS Kool-Aid Man" (the series' 150th episode), where Boomstick finally chooses to move on, while also definitively confirming that Sarge is his father.
The hosts have a tendency to not take the death of characters who have died and came back to life canonically that seriously.
Characters voiced by Takahata101 have a tendency to meet hilariously violent deaths whenever they lose (with the only exception to this being Deadpool).
Episodes involving a Killer Instinct characters always tend to feature two recurring gags, the first being UltraTech being compared to Wal-Mart, and the second being the character of that episode performing a Combo Breaker, complete with the Large-Ham Announcer.
Episodes involving Shadow the Hedgehog have the opponent saying "sayonara" to him right before finishing him off. While this proves successful for Vegeta and Mewtwo, the joke would get gagged when Ryūko's attempt would prove less than successful (with Shadow himself sounding noticeably annoyed at having heard that line again).
Episodes involving a My Little Pony: Friendship Is Magic character have a gag of episodes never actually involving anyone dying. "Starscream VS Rainbow Dash" ends with Dash winning, but she technically never actually kills Starscream due to his immortality. "Deadpool VS Pinkie Pie" has the two becoming friends and abandoning the fight, resulting in a draw where no one loses. "Raven VS Twilight Sparkle" has an Ambiguous Ending where it's left unclear if Raven killed Twilight or Twilight survived with Amusing Injuries. And "Bill Cipher VS Discord" does end with Bill killing Discord, but a post-fight info graph notes that, so long as chaos exists, he will be able to come back.
Whenever a character who appeared in Captain N: The Game Master appears on the show (such as Mega Man or Pit), the hosts will bring up the show's version of that character before Wiz makes it very clear that this is not that version.
Starting from "Aang VS Edward Elric" onward, the series would adopt Avatar's own "My cabbages!" running gag, which occurs anytime a character from the series is involved in the fight.
Season 7 have a recurring gag of characters dabbing at some point during the fight.
In most skits whenever an unexpected result occurs, a bemused character will respond with "Huh, neat."
Most Pokémon videos will feature a Shuckle eagerly waiting to see what special ability/evolution he'll have (as of Gen 8, still nothing). Other Pokémon will either pity him or take advantage of him while he remains blissfully unaware.
In Extra Credits: It was Walpole. (Referring to financier Robert Walpole, who may have very well been the first Prime Minister from his influence.) He shows up in among other things the Crusade videos, and as the assasin of one of the Roman Emperors during the 3rd Century Crisis.
FreedomToons: If a video is about gun control, then expect the gun ban in Australia to be mentioned.
Characters will make a soap-box speech of some kind from atop an actual soap-box, which has a label with an ever-increasing number on it: "ELEVEN SOAPS", "TWELVE OF THEM", "THIRTEEN, Y'ALL", etc.
There was a whole string of episodes of Strong Bad Email that included a random reference to "DNA evidence". A cartoon was eventually produced tying them all together.
Characters, most commonly Strong Sad, over-enunciating or mispronouncing foreign words and names in an attempt to pronounce them properly, like "anime" in "Trogdorcon '97", or "Tove Jansson" in "The Homestar Runner Enters the Spooky Woods".
In every Star Wars video, after Anakin learns he has children, he goes out of his way to tell everyone about it.
For the Spider-Man films, there's a scientist that shows up out of nowhere and demands the characters make sure whatever they're doing won't backfire on them.
Throughout the episode description of almost every Helluva Boss episode, the last few words are always "Things sure do happen".
Every appearance of Adeptus Mechanicus is accompanied by some reference to toasters.
Horus is always referred to as "F*cking Horus".
Karamazov replies to his right hand man's every sentence with "Be quiet, Dominique!"
Dominique really wants to get some sandpaper cigarettes, and no one ever has any or want to fetch them for him.
Every time Fabulous Custodes appear, they want to take over Kitten's position as Emperor's caretaker. Every time, the Emperor is freaked out by the possibility.
Their series on The Escapist also has a few, notably "Eyebrows and the Thugs", supposedly a contemporary jazz band, and "that was the Xth time I died." X is 5 at time of editing.
"Hi folks, Russell here," whenever a crow appears, which is more often than you would think.
TheInvertedShadow's Lyrish series: If there's a TV screen in the room, eventually it will start playing anywhere from three to five random video clips in a row that never last more than two seconds. Material ranges from 2 Stupid Dogs to Starlight Glimmer yelling "I wasfrozentoday!"
Madness Combat: Generally speaking, anyone who smokes onscreen is guaranteed to die. Deimos was no exception.
In Minilife TV, whenever Wilkins, the program coordinator of Minilife Studios, tries to tell anyone his job title, a knight appears and announces it for him.
Characters in Petera Dzive that drink (or sometimes even touch) a bottle of booze will often pass out and wake up naked in inexplicable places. Double Subversion occurs in Episode 16, where the main character Peter wakes up naked and grabs a bottle of booze. He suddenly wakes up fully clothed, only to pick up another bottle and wake up naked again.
Characters laughing by saying "lol", "rofl", and "swa".
"Shush the sheesh!"
In Rick's Minecraft Tales, creepers are ALWAYS drawn flopping back and forth with over-the-top angry faces. Also, the distorted faces people make, the way Nancy goes LO-LA-LA-LA-LA-LA, that thing with the animal floating away aand blowing up, wow, I've lost count!
Weiss being bamboozled by someone Flash Stepping in front of her. First with Ruby, and then with Penny. Also with Weiss, everyone except her being greeted normally — most characters call her "Ice Queen", or "Snow Angel" if they're feeling gracious.
Jaune getting launched into the air, either by catapult, by Deathstalker, or by rocket locker.
Shrapnel: People noting that Reznya should really get a cellphone. Comes to a head in Side of Shrapnel: THIS IS THE WAE, where the entire plot revolves around a Ugandan Knuckles named Goggles trying to deliver one to her from him and his brothers as a thank-you gift for helping them earlier.
Ultra Fast Pony uses "14 Hours Later..." for scene transitions, even where it makes no sense. Also, every time Fantastic Racism comes up in the series, the subtitles assure us "I swear I'm not racist." Every time Mayor Mare gives a speech, a mysterious voice calls from the crowd to heckle her.
In Unforgotten Realms Sir Schmoopy of Awesometon repeatedly casts the spell Flare.
the headcrab being obsessed over Crysis.