Have no enemies
Too many stories are driven by villains. There are many reasons this is bad:
- A plot with a villain is reactive: The villain has a goal, makes a plan, and the hero reacts. You've already given most of the character-building material to an unlikable character; and you've begun by saying that bad people are smart and plan and do things, and good people are conservatives who try to restore everything to the way it was before.
- The villain takes up half your screen time. That's okay in a movie, which is short enough to have one villain from start to finish. But in a TV series, it's wasted time spent developing temporary characters, unless the villain is a constantly-recurring character like Lex Luthor. And if the villain is, that's probably stupid. (There are exceptions. Like Joss Whedon (and MLP wrt Luna), you can turn your heroes into villains and your villains into heroes; then their development wasn't wasted time.)
- If your characters are always fighting villains, we get to see only one aspect of their characters. What is the Scooby Doo Gang like at home? What do they do for fun? Where do they see themselves in ten years? Who knows? Not the show's authors.
- Rooting for one side to win wastes cognitive energy and distracts from the real story. A football game is not art. "Who will win?" is the weakest possible tension to build a story on. The Lord of the Rings is not about whether Frodo will get the ring to Mount Doom. Of course he will. It's about what it will cost him. The finger is in case you didn't already get that.
- It is inherently bad art, by which I mean art that, like a Harlequin romance, makes a person who is exposed to it more dysfunctional. We don't need more stories telling people to solve problems by categorizing the world into good people and bad people and defeating the bad people. They do that naturally.
Have more than one central character
Having the same character be the center of every episode encourages infantile writing and infantile viewing. (This applies only to genres with continuity and character arcs. It doesn't matter as much in comedy, mystery, or suspense; or in genres like action that are supposed to be wish-fulfillment.)
- We have too many stories about the Chosen One, the one with enough Midichlorians in his blood, on whom all else depends, who will defeat the evil empire with his pluck and determination. I've seen too many smart people with Ender Wiggins Syndrome, who waste their lives because they read books that taught them to do everything themselves instead of building networks and learning to cooperate. Yes, the Chosen One is massively popular. So is high fructose corn syrup. We often like things that are bad for us. "Selling out" doesn't mean making money. It means making money by giving people the comfortable self-destructive crap they want, whether that's Big Macs, cigarettes, crack cocaine, Thomas Kinkade paintings, or yet another Chosen One.
- Even if that character isn't the Chosen One, identification makes any series less of a thought experiment and more of a video game. And if the single central character isn't meant to be identified with, he or she naturally drifts towards becoming a daddy figure. The only reason to watch Doctor Who or read Sherlock Holmes is because there's one character who is fascinating. But that one character has to be the driving force behind most major plot developments; and this often leads him to be super-humanly competent and overshadow every other character.
Don't have every main character in every episode
Real people have lives, and their lives don't consist of nothing but endless get-togethers with the same group of people.
Balance exploitation vs. exploration
One of the central problems in control theory, machine learning, scientific research, and business administration, is how to trade-off exploitation versus exploration. Exploitation means using things that you already know. In storytelling, this means invoking things we already know about characters. Exploration means trying new things. This means putting our characters in new types of situations, or examining new aspects of their characters. Non-continuity animation, where each show is an independent entity, is almost 100% exploitation: Assign each character a schtick and a tagline. Show one character doing their schtick and saying their tagline. Play laughtrack. Repeat with another character. You might want to see what crass things Cartman says this week. But you won't write Cartman fan fiction, because you don't care what happens to Cartman, because nothing that happens to him will matter next week.
Exploration is an investment in a character. It means spending screen time adding things to that character that you can use in the future, instead of using the things you already have to get a quick laugh. The Emmys are biased towards episodes that do nothing but exploit, because that gives the biggest emotional payoff within a single episode. But the awards ought to go to the scripts that came before, that built up the character for later scripts to use.
Don't turn flaws into characters
Established writers routinely tell beginning writers to make their characters interesting by giving them flaws. One reason they do this may be because many beginners write what are called "Mary Sue" characters: Overly-perfect characters that are the person the writer wishes to be. But I think the main reason they do this is that we live in a culture infected by Platonism. What I mean is that we live in a culture where people believe in the concept of perfection. "Perfect," unless applied to mathematical objects, is a nonsensical word. There is no such thing as the perfect food or the perfect town or the perfect woman. Yet most people believe, at least subconsciously, in Platonic ideals; and thus that any real-life individual is the Platonic perfect person, plus flaws. (See "Throw out Aristotle" below.) As a result, we have an abundance of flaw-based characters, whose defining characteristic is what is wrong with them.
The characters in My Little Pony are distinguished by what they think is most important in life. What other TV show can you name for which you can easily say what each of the main characters thinks is most important in life? I can't think of any. Their flaws are not generally things they lack, but things they care too much about. And they are only flaws when you take one pony individually. When they work together, their "flaws" become the strengths of the group. This approach to defining characters is the main reason the characters are so likeable.
Give your characters careers and hobbies
Good exploration is character-driven, not initiated as a response to some villain's plot. It is something the characters come up with out of their own goals and career plans. So give them some! Applejack wants to expand the farm's business. Rainbow Dash wants to become a Wonderbolt. Rarity wants to become a famous fashion designer. Twilight Sparkle wants to become the Equestrian equivalent of a tenured academic. Pinkie Pie and Fluttershy don't have careers, but at least they have hobbies.
One goal that sometimes drives characters in MLP:FIM - and not just the villains - is money. Other cartoons take place in socialist paradises, because writers think mentioning needing money will traumatize children or turn them into capitalists. Ponies have to work for a living.
Everything advances character
This was one of Walt Disney's rules. Everything that advances the story must advance character at the same time. If your script calls for an event, and you can't think of a way for every main character in that scene to act that says something about their character, throw the scene out and write something different. None of the generic "the ponies run from the bad guy" or "the ponies play with the bubbles" scenes that you find in previous-generation MLP. If a line of dialogue sounds like anypony could say it, throw it out - it's not conveying information about character, and that means it's not essential to your story. (By contrast, South Park has been running 15 seasons, and I still wouldn't be able to tell Kyle's lines from Stan's lines.)
Characters are more than walk cycles
I've read numerous articles wondering why grown men are drawn to My Little Pony. Even though bronies have given the answer themselves, over and over again, nearly unanimously: Because the characters are interesting.
I think the resistance that everyone who isn't a brony has to this claim, is because animation is so impoverished in character development that we no longer can even imagine well-developed animated characters. People think the cartoons they're familiar with also have interesting characters. But they don't.
The problem is that animation is generally made by animators. And animators are visually-oriented. Animators are very focused on how to visually express a character through their movements. Read The Illusion of Life and you will have the animator's message pounded into you over and over: Draw movements that express that character.
What gets overlooked is two other crucial aspects of character:
- Semantic content
By semantic content I mean what is inside the character's head. What do they want, what do they believe, what do they think about? Hence the importance of careers and hobbies. And also of opinions. The MLP characters express their different opinions about each other, about life, about values, to an extent far beyond any other cartoon characters that I can think of.
Throw out Aristotle
The most frequently-cited rule of writing is one originally laid down by Aristotle: A story must be about a protagonist who is faced with a problem, cannot overcome that problem because of an inner flaw, and changes (although the protagonist may or may not overcome their problem). See Phil Dyer's rules of script structure for elaboration of this formula. It's the basis of stories from Star Wars to SpongeBob SquarePants. It works, but it isn't the only thing that works.
(It also probably isn't what Aristotle meant, since it doesn't fit the famous Greek tragedies of his time. In Oedipus Rex, Oedipus' downfall is due not to a character flaw, but to a lack of information, and/or to fate. People have argued that hamartia should be translated as "mistake", not "flaw". But no translation of "hamartia" can make Aristotle's theory fit the Odyssey or most of the Iliad; so it could be that the entire Poetics was meant to correct, not to describe.)
Some MLP stories can't be described at all by this structure. Some can, but it's like fitting Cinderalla's shoe on to her sisters' feet, Grimm's Brothers-style. The stories are not organized around protagonists overcoming internal flaws; they are centered around protagonists learning the value of friends and family. The plots could more usefully be said to have these structures:
- The protagonist is presented with an opportunity to get closer to one of her goals. As she pursues this opportunity, she realizes the price she must pay to take advantage of it. She decides that friendship or integrity is more important than her goals, and decides to pass on the opportunity. She then gets the opportunity anyway as a karmic reward. ("The Ticket Master", "Sweet and Elite", "The Super Speedy Cider Squeezy 6000")
- Two ponies don't get along, but eventually each decides that keeping their relationship is worth making compromises ("Sisterhooves Social", "Look Before You Sleep").
- A secondary character becomes an obstacle to the primary character, usually while trying to help, by being self-centered. These episodes are often resolved by the secondary characters (who realize what they've done after disaster ensues) rather than by the protagonist, which is a big no-no according to dogma. ("Sonic Rainboom", "The Ticket Master", "Green Isn't Your Color", "Suited for Success", "The Best Night Ever".)
Probably the Ponies' biggest break, not just with Aristotle, but with all of Western literature, is that episodes are often resolved by the actions of secondary characters rather than by actions of the main character. Western literature looks at one individual at a time. You may have a lot of characters, but each one is supposed to resolve their own issues. MLP:FIM has a different message: Sometimes you can't resolve your own issues, and you need your friends to intervene.
Throw out Disney's Law of One Emotion
Another of Walt Disney's dictums, which was also adhered to by Warner Bros. cartoons (possibly excepting Bugs Bunny at times) and is now nearly universal in animation, is that every character must at all times display exactly one emotion. Disney, like stage actors, had problems with people not being able to tell what the characters were feeling. This was partly because Walt Disney usually staged scenes with elaborate backgrounds and did not go in much for close-ups, and partly because the craft of animation was new to both animators and viewers.
The insidious effect of this rule was that the writers learned to avoid writing scenes that called for complex emotions, and eventually learned to avoid writing characters that had complex emotions. The animation controlled the story, instead of the story controlling the animation. This is why Disney had great animation, but could never have produced The Last Unicorn or Grave of the Fireflies.
The MLP:FIM animators said, "To hell with that rule. We are going to depict whatever emotions the story calls for, and assume that the viewers are smart enough to figure out from the context what the characters are feeling." And it works.
Most of these boil down to the same thing: Stories describe and develop characters and their relationships. MLP:FIM is unusually rigorous about expunging anything that doesn't do that, and any obstacles to doing that. As a result, it has more interesting characters than most shows, despite some significant handicaps not enumerated here.
You can get technical tips from the show as well (e.g., show the viewer what the episode is going to be about within the first 90 seconds; introduce a new scene or a new story element roughly every two minutes; do something amusing at least twice a minute). But I'm too lazy and inexperienced to cover that.