''{{Lost}}'' arguably has many of these...

* The most common complaint during its first season is the frustration people had about characters [[PoorCommunicationKills not sharing vital information]] with each other. This is never explained within or outside the show.
* As referenced in the title of the episode, what did the Candidates of Season 6 "die for"? This was never explained.
* It has not been explained, and probably never will be explained, why, in Season 2, the Others did not capture Jack, Sawyer, Kate and Locke when they had them surrounded and disarmed in "The Hunting Party" - only to later offer Michael the following deal: "we will give you Walt and a boat if you bring us Jack, Sawyer, Kate and... Hurley." (They only needed Hurley as a messenger.) This is particularly jarring if you consider that, in ''The Hunting Party'', these facts hold:
## Ben had not been captured yet, and so they didn't need Michael to rescue him yet.
## Ben was already aware that he had a tumor and needed Jack.
## However, Ben didn't know yet who else he needed to kidnap in order to manipulate Jack into doing the surgery in case the brainwashing didn't work. Ethan was probably too obsessed with Claire to pick up on the relationships that had only been set in motion, what, a week? That triangle was ripe pickings. ''And it worked.''
** Blame it on "writing it up as you go along". "Henry Gale" was introduced 3 episodes later, and the writers have stated he was just a guest character, but Michael Emerson impressed them so much that the role was expanded. Ben's story wasn't fully developed, which makes this become FridgeLogic once you know the whole thing.
* The insane trial of Kate in season four. Somehow, the numerous charges against Kate, including murder, assault, and theft, are ''all'' rendered moot because her mother refused to testify against her.
** Not to mention that a lot of these were in the wrong jurisdiction.
* The flash-sideways in general seemed lame, what with taking up half of each episode while ''appearing'' to have zero relevance to the main plots or characters. But "What Kate Does" takes the cake by giving everyone a IdiotBall. Kate doesn't even bother keeping a low profile; she goes right up to a mechanic saying "I'M A FUGITIVE MURDERER!" After being held hostage by Kate when she hijacked her cab, Claire takes all of a second of thought before jumping back ''into the cab'' and traveling around with Kate as she goes to see Aaron's potential adoptive mother. And then she hands over ''her credit card'' to Kate so she can have money on the run... ''What''?
* Ilana's death. Not only did they leave ''another'' character's backstory unexplained with just a ''handful of episodes left'', not only did they go out of their way to ignore her all season despite the actress being made a regular and her being the leader of one of the two main factions, but it also happened in the most idiotic way possible. Ilana was training for the mission all her life, but didn't know that the candidates can't kill themselves (which presumably included mishandling dynamite)? She didn't know how to handle dynamite? Her death serves no purpose: the audience was already aware that dynamite was dangerous from Arzt, and the dynamite could have been gotten rid of another way.
* "Across the Sea." Wasting an entire hour on a supposed backstory for Jacob and the Man in Black that doesn't answer much besides the identities of Adam and Eve, all while the writing gets more and more awkward to avoid having anyone refer to the Man in Black by his name (under the circumstances, there should have been a way to do it that wasn't awkward). Oh, and just after Lindelof and Cuse made such a big deal about killing off Jin and Sun to cement the Man in Black as a scumbag, the entire episode makes him more sympathetic than Jacob!
** To some, he came across as more of a devious spoiled brat.
*** "Oh, and just after Lindelof and Cuse made such a big deal about killing off Jin and Sun to cement the Man in Black as a Complete Monster, the entire episode makes him more sympathetic than Jacob!" This is pretty much the entire point of the episode; after making you absolutely hate the Man in Black and thinking he is pure evil, ''Across the Sea'' shows you that he was just a man, who has been made a villain by his upbringing and the bad choices he made. The episode tackles two of ''LOST''’s biggest themes: first, the idea of man vs fate – Mother decides what Jacob and the Man in Black’s lives will be before they’re even born, Jacob accepts his role, while the Man in Black rails against his, determined to live his own life, but seems unable to escape his fate. The second is the simple fact that bad parents will mess you up – almost all the most important characters in ''LOST'' have serious issues stemming from their relationship with parental figures and even the seemingly godlike pair of Jacob and the Man in Black are no different, they are who they are because they were raised by a twisted woman who stole their lives from them and left them with issues that they never overcame. ''Across the Sea'' represents the central conceit of the show – "you have to let go and move on". The Man in Black cannot let go and is doomed to a cycle of constant misery and suffering, which started with his Mother, while characters like Jack let go of their issues and are able to finally move on in life and be happy. I agree that the episode has some clunky dialogue and could’ve been better, but I think saying it's a waste of time represents a bit of a misunderstanding as to its aims and those of the series as a whole.
* ''The End'':
** Sayid's "soulmate" is revealed to be... Shannon, the woman he had a brief month-long relationship (in the loosest sense of the word) with on the island, as opposed to Nadia, the woman he searched for years to find and lived happily with until she was killed in a hit-and-run accident. [[StrangledByTheRedString Uh]]...?
*** The subtext of the show is that all the characters are obsessing over something, whether its mysterious lotto numbers, long lost loves, destiny, fixing everything, etc, and this obsession is the root of their suffering. This is pretty elementary Buddhism, which the show has been trucking in since the season 2 premiere. In the afterlife, he subconsciously recognizes how he caused himself to suffer, and marries her off to his brother.
** Jack's conflict and reconciliation with his son David is revealed to be nothing more than a figment of his imagination, rendering the entire plot thread (played out of the course of the whole season) completely pointless. While it is explained that Jack has to resolve his own issues before moving on, it does nothing to make the conversations any more than filler.
*** Jack’s relationship with David is extremely important, because it represents the relationship between Jack and his own father, Christian, which is at the centre of the whole series. Like you said, Jack had to resolve his own issues in order to move on and those issues all stem from his relationship with his father – Christian told Jack that he didn’t "have what it takes", he made him feel like a failure and that he would never be good enough, no matter what he did and then, he died, before Jack ever had a chance to resolve these issues with him. This created a deep-seated inferiority complex and obsession with perfection in Jack which leads him to make decisions and choices which affect almost all the most important things that happens in the series. The story of ''LOST'' is ultimately Jack’s story and it can’t end without him resolving these issues; in the flash-sideways, Jack creates the character of David to represent himself as a boy, (hence why other characters constantly comment on how similar they look) as David also feels like he can’t live up to his father’s expectations. Jack, however, tells David what he always wished his father had told him: "in my eyes, you can never fail". This finally closes the loop on Jack’s insecurity because he effectively reassures himself that he wasn’t a failure and that his father did love him, allowing him to finally make peace with his demons and move on.
** If the flash-sideways universe is intended to be a "waiting room before the afterlife", how is it possible for children (i.e. Ji Yeon and Aaron, Sun and Claire's children) to be born after death? [[FridgeLogic And how is it possible for Ji Yeon and Aaron, who presumably died as adults outside the island, to be resurrected as newborns]]?
** The Man In Black's ultimate plan is to get someone to remove a literal "cork" in the island's core (which will supposedly submerge the island), get on a boat and escape... and then what? Not only is his idea half-baked at best (he runs away while leaving three people there who could fix the situation he just caused), but everyone else acts like him getting off the island will mean the death of mankind - even though by the time he's killed, he's just a mortal man. He has no allies outside the island, let alone that he no longer has his superhuman strength or his "smoke" powers - his plan was illogical and not feasible.
*** His plan seems to have been only to leave the island and live as a mortal, even with no allies or money or wtf. Only Jacob tells everyone that he's evil and threatens mankind, but Jacob is either completely wrong about it, or (most probably) simply lying to make people do what he wants, as Ben did during the whole show.
*** It’s explicitly stated that the light at the Heart of the Island is the source of all life in the world and that if it were to go out on the island, it would go out everywhere. The island is a "cork" which holds back a great malevolent force – death itself. The threat to the world is not so much the Man in Black getting off the island, but what he has to do in order to do so. Because the Man in Black’s soul is inexorably linked to the Heart of the Island, the only way he can escape is to get somebody to turn off the light, but doing so will doom the entire world. That’s why the Man in Black is such a tragic villain, all he wants is to be free to leave the island, just like our heroes did all through Seasons 1-4, it’s a perfectly understandable desire; what makes him a villain is the lengths he is willing to go to in order to gain his freedom – he murders hundreds, if not thousands of innocent people and deliberately sets in motion a chain of events he knows could very well end all life on Earth. Ultimately, it doesn’t really matter if the Man in Black escapes the island or not, the threat is in what he does to achieve this; the only reason Jack stops him is because he promised Jacob he would and, more importantly, he’s understandably pissed off that he killed his friends.
* The series ending with Jack/Kate and Sawyer/Juliet. Let's look at these writing decisions:
** The entirety of season 3 was a build up of Jack/Juliet and Kate/Sawyer, and then in one episode both those relationships are just over because Juliet tells Kate that Jack loves her
** Juliet and Sawyer were a convenience pairing done purely so that they wouldn't feel too bad about being the second choice for the people they truly loved
** How are Jack and Kate supposed to be believable as soulmates when they failed every attempt to have a functioning relationship while alive?
** The one key difference between Juliet and Kate that pretty much defines why Jack never could have had a real relationship with Kate is trust. From the beginning, he questions her judgement and feels the need to keep her close because he's afraid she'll mess up if left to her own devices. However, he trusts and understands Juliet almost instinctively. In Through The Looking Glass, he doesn't want her to go back to the beach to fight the Others because he wants her safe, but all it takes is her saying that she has to for him to understand why and agree to let her go. The fact that that's all it took shows how well they understand each other and how much Jack trusts Juliet to make the right decisions and also recognize that she is capable enough to take care of herself and come back to him. They're equals with mutual trust and respect for each other, something that Jack and Kate never have.
** While it was nice to see that Juliet made Sawyer grow up into a mature adult, Jack would never have the patience to do that with Kate. However, what makes them perfect for each other is that Juliet and Jack are both very mature and self-assured people who have and are ready for a healthy adult relationship. On the other side of the coin, we have Sawyer and Kate who are both hotheaded and dysfunctional but love each other, but whereas Kate really never has much character growth on the show, it would have been really easy for the two of them to grow together if they committed to working on their relationship.
** The doctors have a lot in common, the outlaws have a lot in common. What more really needs to be said?