Added Alliterative Appeal: The alliterative sentences for characters in starring images for the 3rd season. It was even lampshaded in the Wrath of Con Blurb. "'Wrath' was the first episode ever to be entirely developed in HD...and inventively initiated illiterated illustrations into the titles!"
Adults Are Useless: Very much so in Pop's case. As well-meaning as he is, his lack of attention and ineptitude often leads to Cub's death (and occasionally his own). Lumpy also falls under this banner in episodes such as "From A To Zoo", and when Giggles' Mom shows up for the first (and probably last) time, she doesn't notice when her daughter's head is replaced with an acorn... or when it falls off, squirting blood in her face.
Always Chaotic Evil: Well not always, but whenever you see birds in this show you should probably start running. Particularly the man-eating ducks.
Ambidextrous Sprite: The Mole's mole, Lumpy's inverted antler, Russell's hook hand and eye patch, and Nutty's lazy eye tend to change from side to side, often in contiguous shots. This is done so often that it almost seems intentional.
Ambiguous Gender: In the show, Flaky's gender is never directly stated and has led to some confusion; Word of God is inconsistent on the subject. Jokes about her gender confusion are sometimes slipped into the show because one of the writers, Kenn Nevarro, finds it hilarious to watch people argue over her gender. Although it has been multiple times stated by Word of God that she is a girl.
Amusing Injuries: Played straight or subverted, depending on how realistically the injury is depicted.
An Aesop: If the series has any fairly consistent aesop, it's household safety. It's quite impossible for parents to claim that the shorts set a bad example for small children, since many of the deaths involve accidents caused by playing with kitchen tools, matches, and the old favorite, not turning something off at the plug. Alternatively, the characters make very certain to turn things off, and it's something else that causes the accident.
And Knowing Is Half the Battle: Each episode ends with a lesson, cheerful thought, or general admonition like "Don't forget to brush your teeth!". Since these were inserted into the Television Series, they now have some distant relation to the lesson you should have learnt.
Art Evolution: The character designs were refined over time, becoming less stiff and a little "cuter." For instance, the characters' buck teeth are rounder, and Sniffles' snout has gotten shorter. The animation has also gotten smoother over time, and the deaths have become more graphically detailed.
Artifact of Death: A cursed idol which appears in several episodes, most notably Treasure These Idol Moments.
Asshole Victim: Lifty & Shifty, and Lumpy when he's the villain. A case could also be made for Disco Bear, Sniffles, and, on the rare occasion that he actually dies, Splendid.
Ass Shove: When Toothy flies off his bike seat in Brake the Cycle, he lands on the seat part minus the seat. Ouch.
Aside Glance: Whenever Handy's lack of hands inconveniences him, he glares into the camera.
Ax-Crazy: Flippy, when his evil side emerges, will kill everyone around him with no remorse.
Quite why he's so utterly insane under his outgoing, if nervous, exterior was explored in the short Operation Tiger Bomb. Let's just say it cemented Flippy's status as the most batshit crazy character in the series.
Flippy. In his evil state, he is able to make almost anything lethal. Double Whammy Part 2 reveals that his good side is equally badass.
Lumpy also gets a few moments where he goes from bumbling to badass. In Doggone It he takes on a giant squid... with a mousetrap. And wins. Unfortunately for him and all involved the level in badass doesn't last. He gets another moment of badass in the halloween episode Read 'Em and Weep when he takes on a demonic Cub, though infection by The Corruption is hinted at at the end.
Bait and Switch: The reason why Random Acts Of Silence dragged out for several minutes until Flippy finally flipped out, despite being provoked with war-reminding sounds repeatedly.
Banana Peel: Subverted in Ipso Fatso. Lumpy notices he's about to step on one, and lifts his foot over it... only to see that the floor also has nails, a snake, spikes, a frayed cord and lava in various places.
The Mole is able to eat things off camera with his mouth covered all the time.
Berserk Button: Don't do anything that reminds Flippy of the war...which happens to be a lot of things, making it more of a berserk 'minefield'!
Subverted in Without A Hitch. Flaky is hugely paranoid that Flippy will go insane and kill her, with three imagined deaths shown. He actually doesn't do anything bad at all. Consequently, she stabs him in the eye and he gets run over by a truck...
Nor should you ruin a library or kill the Buddhist Monkey's plant.
The Ants react very violently whenever Sniffles tries to eat them, but after he destroys their anthill, they proceed to torture him to death.
Even Good Flippy gets this sometimes - he's one of the only characters that have killed someone for revenge.
Also, Lammy. Though she may seem very nice (and she is) she is schizophrenic and will kill you without meaning to whenever "Mr. Pickles is trying to kill you".
Flaky is generally kind, if cautious. However, in Without A Hitch, not only did Flippy not flip out, she flipped out for fear of him killing her and stabbed him, which led to him getting run over by a truck!
Big "NO!": Splendid when he discovers his bread is burnt in Better Off Bread.
while not exactly a "No!", Russell does a very drawn-out "Yarr!" in the episode Sea What I Found. Given the circumstances, it's obvious that he means no.
Bizarrchitecture: The episode where everybody builds Giggles a new house. Because of scrambled blueprints, it ends up being a total funhouse slaughterhouse.
Blatant Lies: The episode description for Happy Trails - Part 1: "Is this really the end of the indestructible Happy Tree Friends?"
Blow Gun: In one short, Lumpy attempts to sedate a baboon mauling Sniffles, but accidentally sucks the dart in when taking his breath.
A Bloody Mess: Occurs in Flippin' Burgers where Giggles gets ketchup splattered on her and Flippy mistakes it for blood, causing him to flip out. Promptly inverted later in the episode, when Flippy dips a fry in Cuddles's blood.
Body Horror: Every. Single. Episode. One that stands out is Cub in Read 'em and Weep.
Bolivian Army Ending: Subverted by accident. "Happy Trails - Part 1" was supposed to be this. At the time, they thought it would be the last ever episode of Happy Tree Friends, and at the end of the episode, the bus falls off of the cliff, with the words "to be continued." However, they ended up creating more episodes, the first of which resolved the cliffhanger.
Carnivore Confusion: Flippy has eaten Mime's remains, and has drunk Cuddles's and Shifty's blood. This seems akin to cannibalism until you realize that Flippy is a bear, and the other three are a deer, a rabbit, and a raccoon respectively; suddenly, the issue becomes that much more complicated.
Casanova Wannabe: Disco Bear. He always hits on female characters, but they don't see to care for him. There have been multiple occasions where, when the ladies he hits on are actually dying, he believes they are dancing.
Comedic Sociopathy: Plenty of it. The characters' disregard for one another's safety is astonishing, though they might just be too dim to realize what they're doing to each other, seeing as they react with plenty enough horror when confronted with another dead character.
Chekhov's Gun: This is one of those shows where if you see some innocuous item or event early on, chances are it'll come back at the very last second to kill the last character who's still alive.
That said, some characters definitely get the short end of the stick more than others. The likes of Splendid, Cro-Marmot and Flippy rarely ever die or even get seriously injured, and to a lesser extent Pop, The Mole and Lumpy die much less often than most other characters. Meanwhile the likes of Petunia, Sniffles, Lifty, Shifty and Cub suffer more than their fair share of the most cringe-inducing, sadistic deaths in the series.
Crapsaccharine World: The world looks sweet and cuddly and so friendly that you can't imagine anything possibly dying in it...which makes it all the more hilarious when the inevitable bloody slaughter begins.
In "Party Animal", Flippy ties some balloons to Cuddles's neck, causing him to float up and get shredded by the fan.
In "No Time Like The Present", Handy gets his foot stuck on the floor with a knife, so he gets a saw to cut a hole in the floor. He cuts right where the fan downstairs is, and Lumpy, who is right below it, gets his head sliced off when the piece of ceiling is detached.
Death Is Cheap: And how. By the end of the episode, at least one character (often more) will be dead or otherwise horrifically injured (e.g reduced to a burnt and bloodied mush after being set on fire and having it stamped out), yet you can guarantee that they'll be perfectly alive and well by the next episode, with no regards to any past deaths. Several of the main characters on the show have died over forty times in total.
Disintegrator Ray: In an episode where characters are competing for the brightest Christmas tree, Lumpy goes a little too far, causing some folks who are exposed to the light to catch fire, and another to completely vanish, complete with charred outline on the fence behind him, a la Hiroshima bomb.
Disney Death: Cub has one in "And The Kitchen Sink" when Pop believes he was decapitated, but he was only buried up to his head in sand. He does actually die later by drowning in the bathtub.
Double Subversion: An example occurs in "Home Is Where the Hurt Is". Handy looks like he's about to pierce his foot on a nail, but moves his foot away from it... just in time to knock the board loose and drive the nail through his skull.
Another case with nails (from "The Wrong Side of the Tracks"): Sniffles finds a rusty nail, gets disappointed, and throws it away. You'd think that Mime would get stung with it... only for the nail to pop Mime's balloon. Then Lumpy reacts to the sound by turning around so fast that the nails on the plank he's carrying get stuck on Mime's backhead, killing him. A nail did kill him after all!
Driven to Suicide: Petunia in Wishy Washy. When she got covered in gunk, after several failed attempts to clean it off, she skins herself with a potato peeler. OUCH!
a popular interpretation is that she was trying to clean the dirt off her and tried the potato peeler since she had nothing else in her kitchen drawer that she hadn't tried yet'
Early-Installment Weirdness: The very first episode from 1999, Banjo Frenzy, has little resemblance to the series other than gory deaths. Most notably, Lumpy is a dinosaur with a personality not unlike Flippy; Giggles is blue instead of pink; and the art style is almost nothing like seen in the actual series.
The most unusual of all is that, after Toothy is decapitated, his head is somehow still alive and bites Lumpy in the foot. You'll never see something like that in the actual show.
Everybody Lives: Nobody dies in "Nuttin' But The Tooth" and "Out On A Limb", but there is still blood. Several other episodes have no gore whatsoever, most notably "Asbestos I Can Do".
"Eyes Cold Lemonade" has Giggles blinded by a falling sign while she and Petunia are working a lemonade stand, and then later Petunia gets her face flattened by the other side of the sign falling. She's killed and her eyeball pops out and lands near Giggles, who's cutting up more lemons, and...you can probably already guess where this is going.
The music video they did for Fall Out Boy's "Carpal Tunnel of Love." Toothy gets stung in the eye by a bee. Then it happens again.
"Eye Candy" is a famous example in which Toothy accidentally jams a lollipop into his eye socket and ends up pulling his eye out. The optic nerve hangs around a tree branch somehow, and then a woodpecker comes and...yeah.
Another example: both Toothy and Nutty get parts of their eyes sliced off by a rogue paper airplane in "Sight for Sore Eyes" and again, the show briefly cuts to their points of view (Toothy's being half forward, half backward, and Nutty's with big red spots in the middle). Russell averts the trope twice in the same episode: first, when Lumpy tries to examine his eyes with a dirty glove, and again when Russell puts in his contact lens with his hook hand.
In "Easy Comb, Easy Go", Disco Bear pulls a hair out of his eye, slices the front parts off of them, AND has a huge mass of hair grow out of them... which he then tries to shave off.
In Blind Date, guess what part of the body Disco Bear gets pepper sprayed to?
A Bit of a Pickle includes Lammy getting taser wires straight to the left eye. Yeah.
There's one in "We're Scrooged" where Lumpy gets a top's tip in his eye. Soon the top begins to drill into his eye.
In the episode "Without a Hitch", Flaky stabs Flippy in the eye with a shard of glass. Then we get to see the world through his messed up POV.
In the episode "Random Acts of Silence", Mime gets a papercut across his eye by Flippy.
Pretty much the basis of "Camp Pokeneyeout". Nutty gets his eye burned by his marshmallow (and by the end he is eyeless), and Cuddles beans Toothy with a rock so hard it leaves a dent in his eye. Ouch. Petunia also gets her slingshot rammed into her eyes.
False Utopia : the world is cute and perfect, but for some reason, the characters always die painfully.
Fantasy Gun Control: There are no guns that fire real bullets anywhere in the series (the only time when a gun shows up, in Concrete Solution, it shoots nails). The writers' reason? "Tooeasy."
Finagle's Law: The only way of explaining some of the accidents. In one episode, Flaky manages to overcome her fear of flying in "Wingin' It", and heroically lands an out of control plane... before a shark promptly chomps her. Then he spits her out... and then Godzilla kills her.
Fingore: Sniffles in Tongue in Cheek, when he gets his hand sewn to the ground and one of his fingernails torn off AND salt on the exposed wound. Oh, God...
Flanderization: Flaky started out as afraid of actual dangerous situations (Water You Wading For, Ski Ya Wouldn't Wanna Be Ya), to half-dangerous situations (Mime To Five, Wingin' It) to almost anything (A To Zoo, Something Fishy)
Petunia started out as a Palette Swap of Giggles, but then gained her Neat Freak tendencies as one of her unique traits.
In the beginning, Fliqpy's killing sprees were just what happened and nothing more, to have those plots turn into more serious, advanced subjects and Bait and Switch moments. He also went from killing everyone, to avoid killing Flaky, to start killing her again on occasion.
Furry Confusion: The episode "Take A Hike" has a grizzly bear slice Nutty in pieces and maul Lumpy to death. There are four anthropomorphic bear characters in the main cast (though none of them appeared in that episode).
Gosh Hornet: Bees are occasionally a cause of death, most notably with Nutty in "Sweet Ride".
Green Rocks: Splendid's weakness is Kryptonut, as seen in "Gems The Breaks".
Groin Attack: A rare female example occurs in "Home Is Where The Hurt Is". In one of the most painful-looking deaths in the series, Giggles slides down an incredibly long banister with nails sticking out of the top. She's groin-ground completely in half. Then there's Nutty in his Party Smoochie, where a yoyo trick ends up hitting him in the crotch, grinding until the friction sets him on fire, and he ends up burnt to death.
It's implied that the killer Turtle from Letter Late Than Never bites Lumpy in his lower areas too, given how he has plaster over both his crotch and behind in that episode.
Grossout Show: Though it's more focused on gory details than the usual Toilet Humour, though this does happen on occasion, such as in Breaking Wind.
Half-Dressed Cartoon Animal: Flippy, Pop, Lammy, Mime, and The Mole all wear only jackets/sweaters. Averted for Disco Bear, Russell and Truffles, however, since they wear full sets of clothing.
Halloween Episode: A bunch of these come out per year that turn the gorn dial Up to Eleven. Only one gets the title of that year's Halloween special. Examples include "Without a Hitch", "Peas in a Pod", and "A Vicious Cycle".
Heat Wave: Swelter Skelter is a ludicrously exaggerated example.
Helium Speech: Cuddles in "A to Zoo". This being Happy Tree Friends, he later impales his eyeball on the nozzle and inflates his other one, letting out a scream the becomes ridiculously high before stopping altogether.
Splendid seems more than willing to get people killed while going after crooks.
High Voltage Death: In the episode "In A Jam", Russel is electrocuted and reduced to ashes by a shorted amplifier at the beginning. Notable for being one of the most cartoony and least gory deaths in the series.
Hope Spot: A lot of these happen. Notably, Flaky in "Mime to Five," after falling off of Cuddles' outstretched intestines and rebounding back up from a trampoline, visibly sighs in relief just in time to get bisected by Cuddles' intestine as she comes back up.
Human Cannonball: Cuddles in Mime to Five. It doesn't end well due to Toothy misinterpreting Mime's instructions, and therefore putting in two barrels of gunpowder instead of two scoops.
Idiot Hero: Splendid. And how! He has little care for his job and kills more people than he saves.
Lumpy, quite frequently. He's the most common "last man standing" of the cast, even at times when he's the cause of it all (although it only happens when it's unintentional).
Incredibly Lame Pun: Pretty much every episode title is one of these, and sometimes the moral at the end is too.
Infant Immortality: Subverted whenever Pop and Cub are shown. Pop actually survives far more often than Cub, though that might be due more to his neglectful parenting.
Instrument of Murder: In Double Whammy Part 2, Flippy fights an imaginary version of himself using instruments from a music store, such as recorders and a harp for bow-and-arrows, a cymbal for a shield (and somewhat of a frisbee), a guitar and Flippy gets knocked out by a trap with a piano.
Lethally Stupid: Lumpy. Good god Lumpy. Remove this Lethal Moron from the show, and the death count will drop by half.
Life or Limb Decision: Being one of the most horrific thing that could happen to someone, the episode "Out On A Limb" of course makes a joke out of it. One of Lumpy's legs is trapped by a falling tree and he spends the next three days hacking his leg off. With a spoon. After finally finishing he finds he cut off the wrong leg. And then the spoon breaks, so he has to use a paperclip.
Made of Iron: Cro-Marmot, Flippy, Splendid, and Lumpy seem to be able to survive situations and wounds that would kill a normal tree friend (due to being entrapped in ice, serving in the military and being a Flying Brick, respectively), making their deaths among the more gruesome ones in the show. In Lumpy's case, he is the tallest of the main cast, and with the exceptions of Splendid and possibly Evil Flippy, he is the strongest. In cases where Lumpy does not die immediately, he ends up getting injured.
Pop and The Mole also seem to die somewhat less than other characters, although this can be attributed to luck more than anything else.
Buddhist Monkey is not part of the regular cast; however, he can take far more punishment than the mookshe deals with.
Made of Plasticine: Somehow, tripping on a staircase is all it takes for Cuddles to be sliced in thirds.
Pop once sliced Cub's scalp open with an electric hair trimmer.
If you're one of the Friends, you'd want to think twice about being near an outward-opening door. Mole actually is killed by an opening door in "Home Is Where The Hurt Is".
It gets turned Up to Eleven with Nutty in "Swelter Skelter" and Sniffles in "Moppin' Up". Nutty gets tripped by Shifty and Sniffles trips in a pool of cleaning liquid. Their brains crack out of their heads. That's it.
It pretty much depends on what makes for a more hilarious death. Apparently, a Christmas cookie, as seen in "Easy For You To Sleigh", is both soft enough to crumble into pieces upon hitting the floor and hard enough to carve through Shifty's ribcage.
Mandatory Twist Ending: In almost every episode at least one character dies horribly. Only a handful of episodes contain no deaths and in those, except for the five mentioned above, at least one character suffers grievous bodily harm.
The Many Deaths of You: Probably half the fun you'll get out of starting a new episode is wondering how many of the characters involved will be dead by the end, and from what.
Medium Awareness: The last segment of "Blast from the Past" re-creates the first official sketch, "Spin Fun Knowing Ya" (in which three of the characters die after being flung off a merry-go-round). Just before Cuddles flies off in this re-creation, he screams what sounds like "Oh my God! I'm gonna die! Not this old death again!"
Mood Whiplash: Most episodes. They often end with a lingering shot of a scene of carnage, or a dead character, then a quick cut to the happy, bouncy theme music and colorful credits.
Naked People Are Funny: Giggles has seen Lumpy's privates in "You're Bakin' Me Crazy" after his towel drops, and in "Let It Slide" when his swimsuit comes off in the pool. note He's always wearing skin-tight shirts, as seen in You're Bakin' Me Crazy, but his typical lack of pants is best explained by Half-Dressed Cartoon Animal
Neat Freak: Petunia, particularly in later episodes.
Negative Continuity: The only way to keep the series going, or else everyone would be dead after about 5 episodes.
Never Trust a Hair Tonic: In "Easy Comb Easy Go", Disco Bear dunks his head in hair tonic after burning his afro. His hair grows back...but in his eyes. Cue Eye Scream when he cuts his eyes off with a razor as he tries to remove the hair from his eyes.
Never Trust a Trailer: The whole show is frequently marketed as an adorable kids' show about fluffy bunnies. Trailers are generally made out of the only thirty seconds of each episode that don't involve somebody getting gutted.
Noodle Incident: In the episode "A Change of Heart," an already dead and mangled Handy is shown in a pilot's outfit next to a dead and rather mutilated orca whale in the middle of a soccer field. No explanation is given (which, to be honest, may be for the better).
Nothing Is Scarier: Sometimes employed for an extra disturbing effect, such as the idol of "Treasure Those Idol Moments" appearing in Lumpy's car after it has caused every other character in contact with it to die horribly. The scene goes black after doing a closeup on the idol in the backseat, with the sound of an unseen car crash.
The trope is also sometimes used to emphasize the already disturbing grievous injuries and deaths seen. "Nuttin' Wrong With Candy" features an already mangled Nutty getting threatened with Eye Scream torture by the vending machine's metal coils and cuts to an outside shot of the fallen machine with blood pooling beneath it.
In "Happy Trails Pt. 2: Jumping the Shark", we don't see what happens to Flaky, as it goes from the other characters glaring at her menacingly before the scene cuts to a sadistically laughing Flippy pounding a headstone into the ground while the other survivors leer from the shadows. The fact that this was the normal, sane Flippy does not help. And neither does the fact that her death was originally supposed to be onscreen, but was cut for being too gruesome.
"Keeping It Real" ends with Evil Flippy sneaking up on Lumpy while he's sitting on a chair. Cue Iris Out, followed by something which sounds like impalement.
Something similar happens in "Wingin' It", which ends with Flaky, wounded but still alive, in the middle of the ocean, and Godzilla roaring offscreen.
Nutritional Nightmare: In one episode, a character enjoys several unhealthy foods in rapid succession, culminating in a deep fried block of butter; he has a heart attack, and in the hospital an X-ray reveals the undigested block of butter blocking one of his arteries.
Offing the Offspring: Many of Cub's deaths are due to negligence on Pop's behalf. However, in Read 'Em and Weep, Pop intentionally kills Cub by beating him to death with a shovel, thinking he was still possessed.
One Steve Limit: Played straight so far, and we already have at least 25 named characters now. In addition to all of the differently named 20 main characters, there's also others such as Lammy, Truffles, Mr. Pickles, Buddhist Monkey, Sneaky, Moues Ka-Boom, Splendont, and a few others.
Only Sane Man: Flaky. Considering that, in the world of HTF, pretty much everything can and will kill you, Flaky is right to be scared all the time.
Reality Ensues: Anytime someone is killed by something that would do the exact same thing in real life.
Several episodes feature comical devices or fantastic vehicles, such as the Driller with a bubble porthole in Hole Lotta Love. You'd expect the driller to dig a hole around the entirety of the vehicle, but it only digs a hole big enough for the body to go through, which unfortunately destroys the porthole and crushes Mole's head.
Red Shirt: Sniffles and Mime become this figuratively and literally in Something Fishy.
Reed Richards Is Useless: Sniffles, resident book worm and inventor, creates some rather useful inventions. Most of his creations backfire when being used by idiots. Although he sometimes tends to come up with incredibly overcomplicated solutions to simple problems, like building a Drill Tank to rescue Cub from a well, despite the fact that a longer rope would have just as, if not more effective. Of course, he isn't immune to the Idiot Ball, despite his intelligence. Sometimes borders on Science-Related Memetic Disorder.
Seldom Seen Species: Cro-Marmot, who could possibly be the only fiction marmot in cartoon history (gophers are more common, though). Note that he never displays any marmot-like behavior. Or any behavior, for that matter.
Self-Deprecation: In the "blurb" version of "We're Scrooged!", during a scene where Cuddles is playing with a robot, the blurb reads, "in the future, this robot will write better jokes than us!"
Serial Escalation: Much of the gore, naturally, but the worst case is when Lumpy created such a huge display of lights for Christmas, that he not only set several characters on fire, but even The Mole had to shield his eyes.
Serious Business: Class Act is the subject of a great deal of debate in the fandom. Major points of dispute include whether or not Toothy was responsible for the fire that burned down the school, and whether or not Splendid, Cro-Marmot and Flippy died in the explosion. For the latter, though, the answer was later confirmed as yes.
Shell-Shocked Senior: Flippy, who even has "triggers" that set him off, from the time that he fought in the W.A.R.
Shoot the Shaggy Dog: Almost every episode, although some prominent examples are the depressingly hard to watch "Ski Ya, Wouldn't Wanna be Ya", where Flaky goes those a lot self-imposed pain just to survive, and "Double Whammy" wherein Flippy goes through a lot of trouble and has an epic battle with his demons, finally overcoming his PTSD...just in time to get roadkilled by a chicken truck.
Blatant one to The Shining in the episode "Aw Shucks!" Lumpy is trying to get a crow out of his house, and the crow seeks refuge in his bathroom. Cue Lumpy (with ax in hand) breaking through the door, complete with a Simlish "Heeerrrreee's Lumpy!"
Single Tear: Giggles does this in "Helping Helps." when she sees that there's a massive tidal wave coming right at her and there's no escape.
Sneeze of Doom: Giggles has one in the Valentine's Day Smoochie that blows her brains out the back of her head.
Soundtrack Dissonance: The series is scored using mostly happy and jaunty music typical of cutesy kids' shows. Even when the inevitable carnage occurs, the music remains upbeat, though it gets mixed in with upbeat ScareChords whenever something gruesome occurs. It is quite jarring to say the least.
Speaking Simlish: They do speak English, it's just almost always rendered barely intelligible. Many times, you can make out actual words, without the consonants (see Medium Awareness above for one example).
Nutty is an exception; most of the time, he just laughs madly or says "Ooooh!". The only time the audience gets to know what he's said is in "As You Wish" note Random Acts of Silence has him whispering something to Sniffles; "I wish I had a scrumptious lollipop!"
Subverted Kids Show: Given the hate mail the producers have received from angry parents, probably one of the most effective in this regard.
Sudden Anatomy: The characters regularly go back and forth between having four-fingered hands and hands that look like oven mitts, even within an episode. They normally have featureless feet without toes, but Lumpy gained two large, hoof-like toes for one scene in "Every Litter Bit Hurts"...and then he was given three toes for a surfing scene in "Wipe Out".
Sugar Apocalypse: Happens routinely."Class Act" has half of the forest critters die when the school they were watching a Christmas play in burns down. The other half dies when the school explodes.
Sugar Bowl: Their tree town is very pretty and happy, a first world forest nation as it were. Pity it goes down in flames every fifth episode.
The Unintelligible: It's very hard to understand a lot of what most characters say, and while they do usually speak in English, it can take a few views to interpret it. This was probably because the show was originally an internet series.
Wham Episode: The ending of the aptly titled Double Whammy, where Flippy is confronted by his evil self. Then Autopsy-Turvy, which turns out to really be Double Whammy: Part 2. Then subverted, since it looks like it didn't effect the status quo after all.
Why Did It Have to Be Snakes?: While Flaky is a coward in general, she seems especially terrified of baby chicks for some reason. Though one can't blame her after "Take A Hike" where she accidentally kills a chick, and a bird snatches her up and uses her corpse for a new nest.
Wingding Eyes: Lumpy gets the dollar sign variation in We're Scrooged!three times. On the third iteration, a dollar sign also appears on his tongue. Lampshaded in the "Blurb" version, where the blurb reads, "$ eyes is a sign of serious illness."
Petunia gets the heart variation in I Nub You.
Mime gets the dollar sign variation in "Mime to Five," followed promptly by unicycles (he'd been wanting to raise money for one all episode).
Yawn and Reach: The Mole does this to Lumpy in Blind Date. Lampshaded in the Blurb version.
You Wanna Get Sued??: Happy Tree Friends made a special Youtube video about Copyright law that will appear when a user gets a copyright strike on Youtube and the user will be forced to watch the Happy Tree Friends Copyright video and take the trivia questions about Copyright. As the announcer would say "Even though Youtube is a free video sharing site, You can still get into serious trouble."