1) Passive voice. This particular problem is one of the most common we see, and it can take two different forms.
First off, the words "was," "were," "am," and "is" are almost always passive verbs, but the changes are usually simple. Take "I was walking" and make it "I walked." Other times, getting rid of that pesky "was" takes a lot more brain power and can prove most frustrating.
The second form of passive voice is very aptly explained in this article by Grammar Girl on Quick and Dirty Tips (I love her by the way. Definitely check out her blog!).
There are very few instances in a novel where words like "was" and "were" have a place and serve a purpose. Use them sparingly. Doing so will make your words and world come to life, and it will push you to be a better, stronger writer. We call it "activating your verbs."
2) Telling instead of showing. I'll admit, I struggle with this one myself when the telling is subtle. However, often times, the submissions we get have blatant telling with words like "felt," "seemed," "heard," "saw," "appeared." Remove all instances of these words in your novel. Don't tell your reader how the character felt, show them by the character's actions, reactions, and speech.
For example: "Sam felt mad." tells us he's mad but does not immerse us in the story nor in Sam's anger. If you wrote: "Sam flung his Biology book across the room and growled like the Hulk after a bee sting," the reader would have a MUCH better idea of the level of Sam's anger. We'd also get a glimpse into his personality and his character.
3) Backstory/Info Dumps. We see A LOT of this situation in the submissions we receive. When saying backstory/info dump, I'm referring to information provided solely for the reader's benefit which does not match the character's normal though patterns. No one walks along thinking, "My mother, Donna Reed, used to ride elephants in the circus but is now content to be a stay-at-home mom." No one walks around their town and thinks, "In the war of 2033, Kernsville became ground zero for a Blitzkreig style attack, which left the town decimated for a decade."
We don't need a history lesson on your character or your world. We need to be immersed in your world and walk alongside your characters. The reader can get to know both through skillfully introduced information--character actions, situations in the world, dialogue, etc.--but it must be consistent with a normal person's thought and speech patterns.
4) Repetition. If an idea has been shared with the reader, it doesn't need to be repeated (unless you introduced the idea a long time ago in the story). Give your readers the benefit of the doubt. They're smart. You've introduced them to your world and your characters, but they don't need to be beaten over the head with the same idea. It slows down the pace of the story and gets annoying. State something that's important and move on.
5) Grammar and spelling errors. Before you submit your work to an agent or publisher, it must be polished. I recommend going through it twice yourself and then sending it off to beta readers and critique partners. Once they've had a look, go through it again yourself at least two more times. If you're not strong in grammar or spelling, make sure to find someone who is and have him/her take a look for you. Nothing turns off a publisher or agent quite as much as an unprepared manuscript.
6) 2nd person. Just don't use it. Unless it's a sh-tick you're trying to do where you talk to the reader (a sh-tick that almost never works, by the way) or unless the story is some sort of interview, avoid 2nd person. Don't talk to the reader because it breaks the plane. Stick to 1st and 3rd person.
7) Leading the reader with questions. We see this problem a lot with submissions too. The character will think a series of questions out loud to lead the story--and the reader--down a certain path. It's not necessary, and it's distracting. The reader doesn't want to be told what to think. Let us ask our own questions. If your character poses a question to him/herself, make sure it's necessary. Make sure you can't convey this information in a more creative and impactful way.
8) Pacing. This one is always tricky, but you should be aware of the pacing for your genre. For example, young adult and middle grade novels need to grab the attention of the reader right away while adult novels can spend a little more time in development. Remember to keep a consistent pace too. Move the story along. Don't spend precious word count on unnecessary things (backstory, info dumps, repetition) that stilts the flow of the story. Action moments should be tense, short, and exciting. Dragging them out bleeds the tension away. One of the best ways to understand pacing is to read books from your genre. Note the amount of time spent on certain things and how quickly or slowly the majority of books in that genre introduce information.
9) World building. If you're introducing any world that's not this world, it will require some suave writing to help the reader understand it. You can't resort to backstory, so you have to use the character's experiences and situations (without resorting to unnatural character thought patterns) to show it to the reader. It's a tall order, I know, but such a necessary balance to achieve. Readers can't be reading along thinking they're in this world and then have a demon pop out to eat someone. It's jarring and unexpected because they didn't even know it was a paranormal world. At the same time, you can't spend pages on a history lesson or explaining all the rules of the world either. It's a very delicate balance.
10) Dialogue tags. There is a debate on this one, whether you should use dialogue tags or character action to indicate the speaker, but I think the consensus, regardless of your stance, is that writers should not overuse dialogue tags. There must be action sprinkled in to give the reader a sense of the characters and their reactions to what's being said. We don't talk in a vacuum. We react to what people say, and we're animated when we speak. I definitely recommend using The Emotion Thesaurus to help show character speech and feelings through action rather than dialogue tags.
There is a big difference in these two conversations:
"I really hate when she does that," Jill said, emphatically. (using adverbs here to explain the speech--also bad, bad, bad)
"I know," Martha said. "She's so mean."
Jill slammed her hand down on the table. "I really hate when she does that."
"I know." Martha touched the base of her throat as she gulped. "She's so mean.
I hope you find these tips helpful! By no means do I know everything there is to know about writing, no one does. But I've been so fortunate to have wonderful editors and critique partners guide me along the path, and I wanted to share what I've learned with you! Happy writing!