Basically, dialogue. But world_wanderer suggested the wording of this (I think), so I wanted to leave that intact.

1) Who knows what?

Yes, yes, I’ve blabbed about infodumps quite a lot in the past, and how authors should avoid “As you know, Bob…” conversations, where the characters recite facts they both know at each other.
Recently, however, I’ve become aware of a subtler form of “As you know, Bob…” in which the author supposedly is able to infodump because one character doesn’t know basic facts about the setting in which she’s lived her entire life.
Yes, read the last sentence again. Don’t worry, I’m waiting.
Yes, still waiting.
Done? Okay, good. And if you still can’t believe that, I don’t blame you.
I’ll use Character A as the person reciting facts, and Character B as the person hearing them. Character A is the Wise Old Mentor figure. Character B is the heroine who’s strangling to death of boredom being at home and wants to leave before her father makes her marry.
Character A: “This is why I’m here.” [Acceptable; Character B probably doesn’t know this].
Character B: “Oh, okay, awesome.” [Yes, it’s sarcastic. See point 2].
Character A: “And this is the date and time when you’re getting married, and this is the size of the church you’ll be doing it in, and this is what the ceremony involves.” [NOT ACCEPTABLE. Character B is more than likely to know all these things, given that she’s lived in the village her whole life and her parents would have told her by what point she had to be married].
Character B: “Oh, great, thanks for telling me!” [By this point I’m laughing hysterically in the corner].
This isn’t clever. It’s still infodumping. It’s still silly. It’s even sillier than “As you know, Bob…” since you have to defy Character B’s initial characterization to get her to pretend she doesn’t know these things. Is she observant? She would have noticed the size of the church. Is she anxious to avoid her marriage? More than likely, the details of date and time are shining in her mind, without the need for emphasis. Has she seen other people get married? Then she knows how the ceremony is performed. Using conversation this way is pure laziness.

2) Modern Earth slang and pop culture references are not always appropriate.

This applies most often when the book has teenage characters, but I’ve seen it with adults, too. People say “Thanks a million” when asked to stand sentry duty when they don’t want to. Things are “cool,” a heroine’s love interest is her “number one man,” and anxious characters who want chatty ones to shut up tell them to “put a sock in it.”
More often than not, this shatters the whole illusion of another world.
Can slang ever be appropriate? Sure. If the fantasy’s light, it almost always is. If the fantasy’s urban fantasy, of course. If the author is deliberately going for a style that imitates one in which slang is used a lot (Brust, who’s heavily influenced by Zelazny, and Cook, who writes noir fantasy in the Garrett series, are two examples), then it practically has to be there. But in each of those cases, slang works with the world, not against it.
I don’t care how hard it is to come up with other ways to say things. If you’re writing a tense historical fantasy, based on the Aztec Empire, with the Aztec-equivalents just getting ready to fight the marauding Spanish-equivalents, quoting Braveheart wins you no points at all. If you’re writing about a primitive village society that’s just developing currency, the merchants shouldn’t understand what other characters mean when they ask for a rain check. (The characters shouldn’t be using that expression at all, actually). A fantasy that’s based on ancient Greece should not have characters swearing with “Jesus Christ!”
It seems simple to remember. But too often, especially in action scenes, fantasy authors don’t bother to look at whether the slang would actually accent their world; they just seize it and fling it in there, wanting the atmosphere of a Hollywood thriller. Well, guess what, you’re not writing a Hollywood thriller. Nor are you writing about teenage characters who just saw the latest Jennifer Lopez movie. You’re writing with different people, and they should speak from within their own culture.
(Oh, and by the way, allow me to actually agree with David Eddings about something for a moment: I find the word “teenager” in fantasy itself a little grotesque. Use “adolescent” if the culture has that as a definition. Of course, they might divide up the life cycles completely differently. I’d like to see that, for once).

3) Characters don’t have to talk past each other just because they’re the protagonists.

Circular conversations, anyone? Big Misunderstandings, anyone? Characters waiting to tell each other the vital secret because “now is not the proper time” and then one of them dies before she can tell it? Characters assuming the worst about each other because of a mispronounced word?
There is no rule that heroes have to stretch the tension in a fantasy book to the breaking point just because they’re heroes. Put them in the middle of a good, solid plot and let them act as they should be acting, given the people they are, and they’ll do a lot for you. But circular conversations are the main method used to swell fantasies—well, that and tons of description—and, once again, I’m just a little tired.
Quite a bit of the time, it doesn’t make sense for these characters to be that indirect with and mistrustful of each other. If they have a deep bond, I would expect Character A to give Character B a chance to say that he was going to the latrine at night, instead of simply assuming he was sneaking off to betray Character A to the enemy.
Fantasy authors also like presenting—well, they say they are—straightforward heroes who take no nonsense and have no patience for the complicated intrigues that may be spinning around them. Yet these same heroes can’t say anything outright. They can’t make accusations against the scheming noblemen even in the presence of people they know and trust. They can’t tell the Love Interest that they love her, already, and end the fluffy angst that’s endured for half the book. They can’t debate with each other about what the best course of action is for confronting the enemy. Instead, they mumble and get upset, blurt things out that they never meant to say, interrupt people on the brink of confessing a secret, assume that anything the Love Interest says means she hates them, and on and on. For supposedly clear-thinking, articulate characters who hate to draw matters out, they’re muddy-thinking, fumble-tongued fools.
Almost all long fantasy conversations that are not the Wise Old Mentor infodumping on someone else, or the final epiphany where all secrets are confessed, are circular. Yet these characters are rarely in high school, and many fantasy authors aren’t in high school anymore, either. Can we please move past this?

4) Words convey tone, and quite often that tone is bitter and petulant.

And again, hey. Meant to be bitter and petulant? Fine by me, although I hope that the bitter and petulant characters grow up soon. At least then I know the author intended it.
Meant to be grand and defiant? Oh, boy, have we got a problem. (And I hope that doesn’t sound petulant, but if you guessed it was meant to sound bitter, you would be correct).
I often see fantasy characters who respond to explanations of motivations, goals, or history with something like, “Fine!” or “I guess so,” or “I suppose I’ll have to do it, then.” This immediately tips my scales of sympathy towards the person who went out of their way to explain [X thing, probably important] here. There’s nothing quite like being a person invested in a conversation with all one’s heart and soul, trying to get across something that you consider vitally important, and getting the verbal equivalent of a shrug, a sullen glare, or a curled lip.
Heroes with comebacks, but no actual conversation, are highly overrated in fantasy. And most of those comebacks are of the sort mentioned above. They’re not grand or defiant. Millions of other people have said them, inside books and out. There’s nothing in them that makes them unique to your character. And if I’m supposed to be feeling sorry for the hero who’s just been saddled with magic and a destiny he never wanted, I would like to be able to think he’s invested in coping with the problems, instead of just saying “Fine!” and retiring to his tent.

5) [Insert Giant Wheel of Cheese here].

This is the place when someone—hero, sidekick, Wise Old Mentor, traitor turned back to the good side, king, or, in fact, almost anyone else in the story except the Dark Lord himself, unless he’s redeemed—makes a warm’n’fuzzy speech, stuffed full of platitudes, topped off with metaphors, with a moral at the end. It will lift troops’ hearts, reassure the hero he isn’t a loser, reconcile the fighting factions, or do whatever else it’s supposed to do. It will probably have the Message of the Story surrounded by arrows and written in Bubble Letters.
It is also the cheesiest thing in the entire, entire book.
People? How many of you have gone through a situation where you’re trying to comfort someone in the depths of grief and don’t know what to say? I’ve had it happen to me, I’ve seen it happen to others, and I’ve heard that it’s happened to other people. At least some of the time, this is a perfectly normal reaction. A Cheese Speech will not always come rolling out of someone’s mouth in a Swiss wedge just because another character is going through a tough time. If anything, the closer the speech-maker is to the grieving character, the less articulate he has an excuse for being, because his friend’s suffering also affects him.
Now, if this character is one who a) would make that kind of Cheese Speech, b) has the kind of relationship with the character(s) in question that would make that kind of speech permissible, and c) has made other Cheese Speeches throughout the story, I think it acceptable. Otherwise, no. Make the character say what he would say, no matter how stumbling and awkward. Then we don’t have to worry about being squashed by the Swiss.

6) DIALOGUE IS MEANT TO BE SPOKEN.

Read dialogue aloud, always. Does it sound natural? Do the sentences come to natural stopping points? Does the punctuation match the breathing pattern? (Usually, when we read written sentences aloud, our breath takes a small pause on the comma, and a longer pause for a colon, semicolon, or dash. When we end a sentence, everyone can tell. It’s extremely very hard to read a sentence over without stopping that way).
That’s why I don’t accept that a character’s every sentence will automatically be long, breathless, and full of flowing descriptions of mountains and water. Characters should speak like people, not guidebooks or commercials for Tom the Slimy Travel Agent.
That’s why I don’t accept that a character will deliver a message in anything but short, rapid bursts if he’s been running or riding for miles. Breathing affects the way we speak.
That’s why I don’t accept that dialogue should constantly be populated with semicolons and dashes. There are a very few fantasy characters who speak so formally they need them. Not many, though. Many of us, particularly in informal situations, speak with commas, question marks, exclamation points, and periods. Listen the next time you’re around your friends, and try to mentally punctuate their dialogue. It may be one long continuous sentence, it may be interrupted by constant questions—of the “You know what I’m talking about?” variety—it may have gestures flying along with it, but it’s highly unlikely to sound like an opening sentence of Jane Austen’s narration.
And that’s why I don’t accept that everyone is going to be equally poetic. It may sound good to say, “She has grace like the gazelle, and the heart of a lion,” if you read it. But try to imagine anyone saying that aloud in utter seriousness with some purpose other than flattery. Cracking up when you try to recite your own serious dialogue is not a good sign.
I’ve said other things before about dialogue, but they’re mainly scattered in other places on my journal, and I was trying not to repeat myself. I’ll link those rants later.