1) Dialogue that is unnecessary
Sometimes we want to fill up space, and so we add a bunch of dialogue. That's how we pad our stories and up our word count. But if the dialogue isn't to a point or purpose, it shouldn't be there. Dialogue should either move the plot forward or help build a character. Dialogue shouldn't be the go-to for a higher word count.
2) Dialogue that is stilted
Unless your character is sitting in the dentist's chair, his speech shouldn't be stiff. Unless he's having dinner with the queen, it shouldn't be too formal. Real human speech has a beauty about it, a natural rhythm and flow that can be difficult to capture on the page. But if you succeed in capturing that beauty, you will have written something to be proud of.
3) Dialogue that is always grammatically correct
Like #2, this third sin has to do with missing out on how speech really sounds. People rarely have a conversation in which every sentence is complete and every verb agrees with every subject. If you enjoy grammar (like I tend to), you may try to tweak reality a bit in your written dialogue and create characters that talk the way real people would talk if this were a perfect world. But unless your story takes place in a perfect world with all perfect people (which would probably make for a boring story), you need to have incorrect grammar in your dialogue.
4) Dialogue that is intended to be accurate to a certain time period or region, but is just difficult to understand
It's tempting to try your hand at Old English when your writing historical fiction, and it's tempting to write accents when you've got a German and Russian conversing in English. But unless you really know what you're doing, that dialogue will be confusing, and more often than not, it'll also be incorrect. The German accent you thought you added won't read as "German" for anyone else. The Old English will sound like a Southern drawl. Just comment that the person has an accent and leave the accent itself to the reader's imagination.
5) Dialogue that sounds the same, regardless of the character
You can learn a lot about a person by the way he or she speaks. Does he say "I" all the time? Maybe he's egocentric. Does she apologize a lot? Maybe she has low self-esteem. Is his tone always condescending? Maybe he's a jerk. And on and on it goes. Proper use of dialogue will allow you to build a realistic character and to show the reader different character traits instead of just telling about them. Instead of saying the woman is mousey, show a conversation in which her mousey tendencies are apparent.
6) Dialogue that is tagged too often
When I say, "tags", I mean "he said", "she asked", etc. Using tags too often or using the wrong tags can kill a good conversation. You don't need a tag before or after every line, especially if the conversation only includes two people. We can keep track of a two-person discussion easily with an occasional tag. Also, instead of using tags, consider commenting on something the character is doing. e.g.
Little Molly slammed her hands on the table in a tantrum. "But I want to go the zoo!"
Her mother shook her head. "We can't, honey. Not today."
Okay, so not the best conversation, but it illustrates my point. Instead of using a tag, use an action that will give the reader an even better idea of how the line is delivered. No doubt Little Molly didn't make her demand respectfully.
7) Dialogue that tries to give too much back story
Let's say that you're writing about two female in the ice cream shop. These friends are having a discussion about boys.
Sally swallowed a cold bite of Rocky Road. "Who are you going to prom with?"
Lisa shrugged. "Well, I'll tell you for darn sure that I'm not going with Dillon. As you know, he did break my heart when he cheated on me with Ann last month. As you know, he took her to the same movie I'd wanted to see but he couldn't take me because he claimed to be 'busy'. It turns out they'd been seeing each other for a few months behind my back."
See my problem with this? The information about Dillon may be necessary, but Lisa wouldn't be saying it right now. She probably wouldn't say it all to Sally, since apparently Sally already knows what happened. There are other ways to include back story. Don't be too worried about putting it in all at once. Try to work it into your story in such a way that the reader doesn't feel she's being lectured about character history. Maybe Lisa ought to run into Dillon on the street, and the reader must glean the truth from the conversation that ensues. Maybe Dillon accidentally texts Lisa when he meant to text Ann, and Lisa starts obsessively thinking about the situation.
<b>So, who has an eighth sin to add to the list?</b>