Ooooooooh! This is a lovely topic!
Okay, I dole out a lot of writing advice on the regular and I have a handful of things that come up almost every time. I'll pair it down to four:
Adverbs:
Adverbs are wonderful words, and unfortunately they can be some of the most difficult to use in first person and in third. Why? Because often, they're useless. So what do I mean by adverbs? Well, words ending in -ly. The biggest problem I've seen in every novel I've given feedback for, is the use of words that aren't required and -ly is the biggest offender. For example, here's a sentence:
"She tiptoed daintily into the room, barely making eye contact." seems like a harmless and normal sentence, right? Well, not so much. there are two words here that not only aren't needed, but also take the action away from the reader (so instead of being with the girl, in this case, we're watching her). Here's the fix:
"She tiptoed into the room with her eyes fixed straight ahead."
I've used two instances of -ly here. The word "Daintily" was erased entirely because it didn't make sense. She's already tiptoeing, and tiptoeing is dainty, so why would "daintily" even be required? Exactly, it's not. So I trashed it. The second instance of -ly is in the word "barely". In this case, I had to restructure the sentence after deleting it. I did this by introducing an -ed word that puts us more in the now, "fixed" being the operative word. This way, we still know that she's not looking at anyone, but I don't feel out of the action as much. These are two basic ways to fix problems that adverbs (beautiful as they may be) can introduce. This leads me to my next point.
Passive Voice:
This is related to adverbs in that, adverbs often cause a novel to sound passive. What is passive voice? It's a method of describing an event without actually getting into the nitty-gritty. You're using 'tell don't show' versus 'show don't tell'. For example, I'm using passive voice right now. Notice all of the -ly words I can get away with? That's because we're having a conversation and my descriptions aren't being used to paint a picture, but to paint a point. To that end, when you're writing a novel there are several ways passive voice can happen and more than adverbs can cause it. Example:
"Jerry was thrown the ball and he caught it while he was running. Then he was tackled at the last line but didn't make the goal."
In this illustration, you can see that I just told you that Jerry was tackled. Not exactly riveting, right? That's because it's done in a passive tone. I'm giving you a play-by-play of the action, rather than showing you. The words that converted this to a passive narrative in this case are: While, Was, Running, But, and Then. Now, words ending in -ing can turn your voice passive, but it's all in the manner of how it's used. This is something you have to determine as you edit, if an -ing word can be replaced with an -ed word and still make sense. If it is the case that the -ing word can be replaced, even if you have to change up the sentence, do it. -ed keeps readers in the now.
Likewise, telling you about how Jerry was tackled, I employed words in a passive way, such as combining usages of 'was' and 'while' as if I'm recounting the story, rather than the author is experiencing it. How do we fix this? Well, -ed words make a great start. To fix:
"Jerry caught the ball in mid stride, the line in sight. He felt his legs go out from under him and a shoulder in his back. He pitched face forward into the grass, a mere inch away from the line."
Now, this is what the action looks like from Jerry's perspective. Pretty different right? So when writing a story from a character perspective, it's important to include details and stay in the now. (Also, as I keep liberally handing this out: Editminion is a great tool for identifying narrative issues) this also leads into my next point.
Show don't Tell:
Boy if I had a dollar for every time this happened Bill Gates would be calling me for financial advice. I did illustrate the show don't tell rule pretty clearly above, as Passive Voice acts for a medium of telling rather than showing. That second paragraph I wrote illustrates the opposite. However, I do find this bit important to stress. When you skip details in order to hurry up and write the next scene, it harms your overall story. Now, in most first drafts this happens because you just need to get the story on paper and that's understandable. However, when editing it's important to catch these instances where you've breezed over a description that is otherwise important.
Also important when it comes to descriptions: Don't overdo it. I know I keep using him as an example but Christopher Paolini did this to the point of absurdity. I was the age group he was writing for when he wrote his books and I couldn't manage to make it through his work, because he over described details that were unimportant. If the scars on your cousins right thumb that he got while playing hooky from being a farm hand, in a boring town on a spring day in September, turn out to have The One Ring AND a TARDIS inside them, then they're relevant. Otherwise, skip it. Describe what's important and what moves the story along and just make sure that we, the readers, can see it as well.
Character Soup:
This one is really rough. Particularly in YA novels that have large casts often of high school kids or many dystopian groups of some kind. If you're going to have a large group of people, start with a handful first and don't rapid-fire-introduce everyone you think is relevant within the first chapter. That's way too much to manage for a reader. For example, your main character is a high school student and he's meeting his friends on the first day at a new school. Often this happens:
"Joey Meyers is my best friend. He's meeting with Jeanie and Jamie, they're twins and their parents Jocelyn and Jacob love "J" names. They're going to the mall after school with Tina and her girlfriends, Sarah and Tammy. I wish I had friends to go to hang out with after school, but Joey didn't invite me to go and I can't invite myself."
Notice how many named characters we have in that paragraph? There's eight, and of those eight, how many of them are we going to get to know? If the answer is none of them, then none of them are relevant to mention and Joey' after school mall trip can be called what it is:
"Joey is going to the mall after school with some friends, and I'm not invited."
Or, if some of those people are relevant, than introduce them, or if all of them are relevant, introduce them! But don't do it all at once. You could have one part of your story where your protagonist meets Jeanie and Jamie and gets to know them before moving on to other characters. Introducing us in a staggered way, means that we get to learn the people we're going to be interacting with and we don't get confused as to who is who. Which is the biggest problem with character soup and introducing too many people at once, it confuses the reader. Let us get to know people first or introduce them in such a way that they're memorable, and then let the other characters trickle in. We'd hate to miss out on good character interaction because we got too confused as to who were were talking to.
And these are some of my points. I hope this helps out other authors, as these are the issues I've run into most often when offering feedback. Cheers, everybody!