You didn't give a character a skill he will need; you didn't expound enough on his hatred to make the reader believe he would betray the cause for it; you didn't make it clear what the gem's powers are; you didn't give the law that will govern the consequences of his actions.
Sometimes you didn't foreshadow the right questions.
Or, of course, you answer the wrong ones, but climaxes are hard enough to devise that it's easier to go back and establish the right questions even though it has to be a lot of places.
If, for instance, the heroine stumbles on the city that created the enchantment that is such a problem at the climax, there has to be a point -- or many points -- where she wonders what the source of it was. If she's preoccupied solely with containing it -- after all, its history could be what it pleases, it's its present antics that are the problem -- something else has to hint at it. (And probably she has to have character development that leads to her realize that, hmm, that "the past is not dead; it is not even past.")
Readers aren't going to wonder about questions that the writer doesn't raise (at least, if the writer manages to draw them into the story properly) and so won't be curious for the answers.
