Lost is mostly ed for its mysteries and sense of intrigue (not to mention its infamous ending). And while the mysteries were certainly captivating, they would have meant nothing if it weren't for the stellar cast of characters trying to figure everything out.
Lost contained a truly spectacular cast of characters (played by a spectacular cast), and the strong writing on display ensured that everyone received some strong character development. Well, mostly everyone. While most characters were given strong and meaningful arcs, others were unfairly cast aside and had their stories cut painfully short. Either that, or their story went in a rather disappointing direction.
Walt
Walt's story is perhaps the most disappointing in all of Lost. He enjoyed some good character development throughout the first season, eventually growing to love and trust his estranged father, Michael. Things began heating up in the latter half of the season when it was implied that Walt had some type of special power - a "power" that became even more important in the first half of season two. Unfortunately, not much came of this story arc, and Walt just sort of disappeared from both the island and the show.
Shannon
Shannon was one of the most hated characters throughout the first season. She was spoiled and often bratty, and she rarely ever helped anyone but herself. Even when she did help, she often wallowed in self pity and pushed others away with her negative attitude. Things started to get better at the beginning of season two, and it looked like Shannon was finally getting some semblance of sympathy and character development. And then she was accidentally killed by Ana Lucia, and that trajectory came crashing down.
Hurley
Hurley was a widely beloved character, but if there's one issue with his writing, it's his significant lack of character development. Hurley was often used as comedic relief, and while he was given a unique backstory (complete with his bad luck and his association with "the numbers"), this proved meaningless upon the show's conclusion. Hurley doesn't really change throughout the course of the story, and while he is given the role of island protector, he didn't change in any significant way to deserve it. Season one Hurley could have very well been cast as island protector as well.
Claire
Unlike Hurley, Claire goes through some significant changes throughout the story. Unfortunately, some of these changes proved controversial with the show's fanbase. It seemed like Lost didn't know what to do with Claire after season two or three, so she literally just disappeared from the show.
When she returned, she was an evil version of her past self, living off the island like a second Rousseau. It was a weird creative direction for the show and the character to take, but the audacity must be commended.
Locke
Locke had a fantastic personal story, but like Hurley, he didn't change much throughout the course of the show. Minus a brief stint in season two when he lost his faith in the island, Locke remained firmly devoted to the island and its magical properties. Then again, his lack of change is precisely the point. Locke was blindly devoted to the island, and this blind devotion eventually resulted in his death at the hands of Ben. Locke was easily manipulated, and it made him the saddest character on the show.
Desmond
Desmond was a fan favorite, and his character arc remains one of the show's best. Desmond begins the story as a coward, willing to run away from anything that even remotely threatens his happiness and personal comfort. By the end, he is a tough and commanding man willing to do anything to protect his family - like nearly beating Ben to death after Ben pulled a gun on Penny. Along the way are Desmond's adventures with time travel, which proved one of the most unique aspects of the entire show. There's really no going wrong with Desmond.
Ben
Ben Linus is the show's biggest antagonist (outside of The Man In Black), and for five seasons, he played the megalomaniacal and psychotic villain to perfection. Ben was perhaps the most selfish character on the entire show, and he wasn't above things like murder - and even total genocide - to get what he wanted.
However, Ben is greatly humbled after meeting Jacob and being told that he's a nobody, and this fateful meeting greatly changes his outlook on life. It doesn't redeem his actions, but it does provide him a great character arc.
Kate
Aside from maybe Ben, Kate is the most selfish character on the show. Throughout the first three seasons, Kate was mostly concerned with protecting herself and doing whatever it is she wanted to do, and it often resulted in her being ostracized from the group (the fact that she was a fugitive certainly didn't help). However, that began to change after she got off the island. Kate became much more maternal by taking care of Aaron, and she eventually returned to the island for the sole reason of rescuing Claire. From selfish to selfless, Kate underwent an excellent character arc.
Jack
Despite Lost being an ensemble show, Jack Shepherd is widely regarded as the "hero" of the story. He becomes leader of the castaways (despite his reservations), and he often takes charge when it comes to defending the castaways from external threats. However, he remains annoyingly stubborn and often refuses to consider other viewpoints. This changes after Locke's death, as Jack turns from The Man of Science into The Man of Faith and leads the charge in protecting the island from The Man In Black.
Sawyer
While Jack is the "hero" of the story, many fans consider Sawyer to be the greatest character on the show. He certainly enjoys the greatest character arc. When Lost begins, Sawyer is a belligerent, racist, selfish, and violent outcast who goes out of his way to annoy people. He thinks he deserves punishment, and making others hate him is the best way to receive it. However, Sawyer undergoes a terrific character arc throughout the first five seasons, and season five is mainly devoted to his strong leadership and heroic actions as LaFleur.