The insufferable love quadrangle between Jack, Kate, Sawyer and Juliet. Five seasons packed with plots, subplots, twists, characters, sci-fi, horror, supernatural and all kind of crazy stuff, thousands of questions to be answered in just one last season... and they keep giving a lot of screen time to that stupid love story? Especially during the second half of season five, and especially with the finale.

The way they killed Libby off without revealing virtually anything about her

The way those wretched flashbacks about Jack's tattoos in "Stranger in a Strange Land" seemed as if they were written at the last minute so they'd have a flashback for that week.

Kate. Just Kate. She has almost no connection to the Island mythos and seems to exists solely as a trophy for Jack and Sawyer to compete over. And she sucks away precious airtime from far more likable and interesting characters. Furthermore, her centric episodes tend to be the worst of a given season.

Eko's death... talk about a lame end to a promising storyline. Though it was caused by the actor leaving.

Faraday's reduced screentime as season 5 went on, culminating in a rushed treatment of his storylines and his death.

Nikki and Paulo. I mean, come on.

Desmond not having any storylines in season 5.

In a single Season 5 episode, they kill off all the background characters who had been around since the beginning. All of them.

Kate and Jack... I don't hate them, they just aren't that interesting. I was kind of glad that Sawyer started becoming the leader while Jack was captured, but then he came back and ruined it all.

Half of the main cast of season 6 (such as Ilana, Jin & Sun) getting almost no development all season before abruptly dying.

Kate was shot in the chest in the episode "The Candidate" and wasn't one of the three characters in the episode who died.

Jack, the main character of the show, is its least interesting character.

The creators claiming the events taking place on the Island would have a rational explanation and then blatantly ignoring it.

Generic complaint about the finale.

My biggest complaint is that sometimes the show had problems juggling so many different characters. Several were shoved aside, and others had abridged and/or rushed storylines.

"The Candidate" killing off three characters who had done nothing all season, and also killing off mostly non-white cast members, including the only non-white woman on the show.

"Across the Sea", the Jacob/Man in Black centric flashback episode, was written like really bad fantasy fan fiction. Even worse, with an episode THIS close to the series finale, it provided more questions than answers, and the few answers that were included were executed poorly (like the donkey wheel, the Adam & Eve skeletons, and the source of the island's...erm...essence). Easily the crappiest Lost episode in a long time.

Whatever happened to that spy plane that found the island for Penny but was never seen or heard about again?

All of the mysteries exist for the sole purpose of getting you to tune in next week. Ultimately, nothing means anything.

"The Others" turning out to be something mundane. Unlike their creepier debut in the middle of season 1. And the less we talk about Ethan the better.

We never get to know the Man In Black's name.

Charlie's death in the season 3 finale, considering there was seriously more than one way he could have saved his own ass from drowning.

Out of all the characters that have died, I think we're up to four whose deaths were actually relevant to the plot.

All of season 3, save the last five or so episodes, in which nothing happened.