One of the hallmarks of poor character development is contradiction, indeed internal consistency is paramount in writing as a whole, not just for characters, but today we're going to focus on characters. Now, don't think that I'm saying it's poor character development for an otherwise "nice" character to develop into a greedy, sadistic or otherwise "not nice" person, that's fine and dandy. The trick there is simply providing suitable reasons for a nice character to become a not-nice one. Do bad things happen to them? To their loved ones? Is their worldview shattered in one single event or do a long string of events slowly wear them down? There's plenty of reasons for a nice character to become a not-nice one.
What becomes a problem is when your character begins with, or develops contradictory traits. That's not to say all contradictory traits are automatically bad, humans and any fantasy species whose brain is remotely humanoid are often contradictory. Doublethink is a hallmark of the human condition, consider those times you've heard a racist or a sexist say "But by best friend is a *category I hate otherwise*." As natural aspects of humanity, it is not unsurprising for a character to possess a few of these. Emphasis on few. Writing becomes exponentially worse the more of these traits a character possesses. Of course, as I said internal consistency is important, and it is not unusual for a character to be one way in one situation, and another in a different one. This is not contradictory.
The worst part of contradictory traits is simply that it closes off routes for development. Take the Sims for example. In it your character can possess only 5 traits, and with a special perk, a 6th. Yet there are hundreds of traits in the game. You may over time, move from one trait to another, and this is good. Humans are limited. They can only be so many things at any given time. A poor example can also be found in the Sims, the Lifetime Rewards. Through them your character can gain any number of "special traits", and keep gaining more and more and more until there are none left! Which is precisely the problem. The more traits you gain, the less remain to be gained, and thus, the less room your character has to become developed.
Development does not always mean improvement, nor does it mean change. The more a character develops, the more dead-set in their ways they can become, just as the more developed a character becomes, the more open to new ideas they can be! Going from being casually negative towards a certain group of people to being outright bigoted, even violent against them would by most be regarded as NOT an improvement, not even change by some. And yet, it is. There is no reason for a character to always become a better person, while those stories are heartwarming and often make for good moral teaching stories, a character who does not become a better person is not therefore, a bad character. You know the kind, the guy you "love to hate". They're a jerk, they run their mouth, they're racist, they're arrogant and may even become more and more of those things as the story progresses. This does not by any means prevent them from being well written or good characters, they're just not ones you like.
But I digress. The character who becomes more set in their ways, does not "grow" in the usual sense can and often is a great character to write, but it can also be very poorly written. It's not about saying "no" to everything and never changing or growing or improving. It's a certain sort of self-righteousness that when presented for an opportunity for growth or change, they scoff and turn away, and successively over time the idea that they are always right becomes more deeply rooted as they see specific instances of people "trying new things" failing and getting hurt for it.
So how is this related to contrary traits? Well, even though Bob the asshole here never gained any, his character still developed. Chances were presented to him and while he gained nothing new, he didn't lost anything either. A character with contrary traits is prevented from following through on developmental opportunities. They're nice, and they're mean. They're an extrovert, and a loner. They're generous, but envious. These kinds of characters, and I'm sure you've seen them, when presented with opportunities, simply can't take them up. They've already gained what that situation would provide them. Which means they don't grow, they don't develop, they never have a change to change, improve, worsen, nothing, they already are.
This is perhaps, the biggest marker of the Mary-Sue and bad writing: the character who never develops. When presented with things, they've already got what they need. The situation is but a minor irritation and at the end of the day, they're right back to where they began. They're not nicer or meaner, they're not more or less generous, they're exactly as they were before.
And please don't excuse your Mary-Sue with the reasoning that they are so unfathomably inhuman, so unrelatably powerful, so unimaginably talented that we, the poor reader, simply can't grasp them. Well, if your reader can't grasp them, why the fuck are you writing about them? If the reader can't connect with the character, they're not going to be much interested in their not-adventures, their not-trials and their not-tribulations. Humans are naturally contradictory to some degree, but when that degree becomes so extreme that the character possesses nearly every trait, exists at every angle, thinks from every perspective, it just becomes boring.
It is not what a character can do that makes them interesting, but what a character can't do. How a character manages to deal with difficulty or challenges is most interesting when they do not already possess the skills/tools/abilities/talent to handle the situation.
So next time you write or draw a character, don't think about what they're good at, think about what they're not. Because that is what will make them interesting to the viewer/reader. To be deep: It is in our limitations that we find success.
Listening to: Brian Tyler - TFP Soundtrack
Reading: MMOC forums
Watching: History of the USSR put to Tetris
Eating: Cup o'Noodles
Drinking: Cup o'Noodles