Angst? What Angst?: Beeza and her species in "The Ball" had to constantly move from planet to planet to escape from Buster, who practically destroys not only the planet but all the hard work they had done in it. Sure, they treat it as routine, but it gets worse when she mentions that they have no planets left and they will just keep flying until they run out of fuel or food, whichever comes first.
Anvilicious: The fact that Hater's romantic overtures for about six episodes to Dominator are leading to her trying to kill him. Dominator's hitting the audience over with how you shouldn't hit on a girl when she's not interested.
Awesome Ego: Lord Hater definitely, but to a lesser extent, Emperor Awesome fits this by trope and even more so by trope name.
Base Breaker: Wander, the lead protagonist, has become this. Many fans love him and find his antics entertaining and fun, but others find him incredibly annoying and uninteresting compared to Lord Hater.
Bizarro Episode: "The Void", in which Wander and Sylvia visit a white space (think Michael Crichton's Sphere) where anything either of them dream up comes immediately true, and the rules of logic and common sense do not apply. It's pretty much non-stop insanity from start to finish.
The plot of The Battle Royale has stirred up the fanbase. Some have vocally expressed disgust of Wander trying to fix Hater up with Dominator after discovering her secret since it feels that Wander is using her as a tool to make Hater good. Others have looked over this problem and have faith that Dominator will still be her own person, and vicious threat to the galaxy, and it will lead up to Wander's own character development.
The ending to the episode, too: Some are excited to see what will happen now that Hater has a crush on Dominator, other's are calling it a cop out and are worried the romance will take over the show.
Crazy Awesome: Wander, continually saving planets by basically acting like a complete goofball. Highlights include stopping a feud in "The Good Deed" by acting out an elaborate courtroom scene by himself and literally confusing everyone into submission, to saving an entire planet by wining a dance off in "The Party Animal".
In "The Boy Wander", Dr. Screwball Jones gets this with his tickle machine. Subjecting Tickle Torture on unwilling subjects, and then overpowering Hero!Wander in a fight? Deliberate Nightmare Fuel and suspense. Doing it while singing a polka Villain Song? Hilarious.
In "The Black Cube" when everyone where confessing their bad days, one of the gang of teenagers mentioned that he is suffering from a distinct case of ennui...but then he admitted that he use words he doesn't understand to sound important.
Peepers too. Although he is The Dragon and quite ruthless, he also earns sympathy for being a cute little eyeball that is good at his job and would have conquered the galaxy earlier if not for Hater's terrible leadership. He also spoke for so-called "nerds" everywhere in his Hannibal Lecture to Emperor Awesome.
Lord Dominator is getting a lot of this based on the previews alone. The reveal that she's actually a Green-Skinned Space Babe under all that junk probably helps a lot.
The Tumblr fandom was excited for Westley months before his episode aired. When the episode finally came out, the fans were not disappointed.
Heck, all of the Watchdogs could count as this since they get so much love and attention from the fandom.
Some background characters are popular in the fandom because of their design. Most notably, the pink and green alien who appears at around 0:19 in the theme song has quite a bit of fan-art for a character who has never appeared in the show.
When an airdate for "The Enemies" was announced, Brad Starlight got a small fan-following as well.
Lord Dominator, and how!
Dr Screwball Jones. Being voiced by Weird Al certainly helps.
The Revealthat Lord Dominator is actually an Adorkable girl not unlike Wander in terms of personality has resulted in many people shipping Wander/Lord Dominator or even Hater/Dominator
After the summary for The Battle Royale came out, Hater/Dominator spiked in popularity. It helps that Wander ships it in canon.
Fanon: Wander is often called a "star nomad". McCracken did call Wander "nomadic" and there's a star on his hat, but he's never been referred to this in canon.
"Funny Aneurysm" Moment: An odd inversion. In "Battle Royale" Wander defeats Dr. Screwball with a "Bang!" Flag Gun, prompting the latter to enact an overly dramatic death scene. Hilarious? Yes, only it recalls when the DCAU Joker died due to a Brainwashed and Crazy Robin shooting him with a lethal version of the gun. Bonus points for Wander being in his "grim and gritty" heroic mode.
Inferred Holocaust: While we don't see an owner in "The Ball", Buster does have a dog tag with his name on it. So Beeza and her people have a new home until Buster's owner (or someone else) finds him and decides to give him a bath/flea dip.
Commander Peepers. Evil bad guy and ruthless second-in-command? Yes. Butt Monkey whose boss is a Man Child and has only recently come to appreciate him? Also yes.
Lord Dominator in "the New Toy," due to the fact that she has an Abhorrent Admirer in Lord Hater. Although she invokes Flaw Exploitation and Distracted by the Sexy to take his car keys and use a weapon against him and Peepers, she isn't happy that a walking skeleton is obsessed with her.
The Black Cube of Darkness may be a soul-stealing Eldritch Abomination, but after the events of "The Black Cube" it is easy to see why he turned out that way. He is staying at a run-down apartment that he can barely afford rent because he has a crappy job at a fast food joint, his girlfriend dumps him, his lack of hands makes even the simplest of tasks a chore, and everyone either hates or fears him because of his power. Even when he saves one of the punks harassing him, a mob forms to attack him. Luckily, things seem to be looking up for him in the end.
From "The Picnic", Wander carefully scooting down the side of the crater.
Lord Hater is a reaction face gold mine.
Wander taking selfies.
It's quite common on Tumblr for fans to take clips from the show (that involve notable movement) and sync them up to popular songs or pieces of music. Usually the first shot of Wander in the opening, where he's walking with changing backgrounds. This meme even resulted in an official Wander over Yonder promo set to Quest to the Hall of the Mountain King
Photoshopping Lord Hater's face onto gifs of other skeleton characters.
Moe: Wander, all of the Watchdogs, and even Sylvia all have their adorable moments.
The song "The Commander's Last Command" from "The Axe" contains such silly lyrics as "a sandwich took our planets". Does this prevent the song from being any less heartwrenching? Hell no.
The episode "The Black Cube" is about a floating black cube's spiraling depression. The episode has a sense of silliness given the main character but the episode's continuous Humiliation Conga and oppressive atmosphere successfully makes it rather sad.
Private Westley from 'The Little Guy' is extremely popular despite only being in one episode (aside from one cameo in the first season finale.)
Beeza from 'The Ball'
The two-headed fruit bartender from 'The Bad Guy' has a ton of fanart, moreso than any of the other characters in that episode.
The BLACK CUBE from 'The Fancy Party'. It's eerie floating and pseudo-Latin gibberish (as well as the fact that the whole area distorts when it speaks) makes it give off the feeling of being an Eldritch Abomination, making it one of the most memorable of the villains at the party.
The drill sergeant and Giuseppe the Watchdog tailor in "Eye on the Skullship".
Paranoia Fuel: In "The Lonely Planet", we meet a planet who is not only alive, but can control the weather, plant growth, and plate movement, and use those to harm people. How do we know the other planets in this universe (including ours) aren't like this?
Rooting for the Empire: As of "The Cool Guy" and Peepers' Hannibal Lecture to Emperor Awesome about how "nerds" get things done in the world, at least one viewer is rooting for Commander Peepers to spread evil through the galaxy.
Some Anvils Need to Be Dropped: With the constant plotline of Wander trying to pair up Dominator and Hater, and Hater constantly flirting with Dominator, it's clear that you shouldn't pair up people simply because of a few similarities, and that a guy should learn to take "no" for an answer the first time.
The opening sequence, showing off the sheer variety to every character and world introduced in the show, repeatedly cycling through different backdrops and aliens- each of which feels different to the last. It's a real testament to the creativity and imagination behind the show.
Wander and Emperor Awesome's dance off in "The Party Animal" has some very fluid animation.
Wander and Screwball's duel in "The Boy Wander" has amazing lighting usage that lends a sinister air to this otherwise silly fight scene.
Hater's and Dominator's fight in "The Battle Royale" is brilliantly animated and action-packed.