- Flashbacks. It's easy to understand why writers tend to like writing flashbacks. It's an easy way to let the reader in on a character's backstory, and it gives the writer a chance to explore a character and cash in on an event that's probably emotional, exciting, or both. The problem is that, for the most part, readers don't like reading them. From the reader's perspective, a flashback is most likely an interruption of the current action in order to look back on an event that, in all honesty, they could do without. And besides, there's a good chance it'll be written in all italics--yuck. As the writer, if writing that flashback makes you feel better, go ahead and do it. It might help you gain insight on a character or event. And then, in almost all cases, delete it. It's probably not necessary, and this might be a case where a small piece of telling to get your backstory across is better than an entire scene of showing. (More on showing vs. telling here.)
- Dream scenes. Much of what I said about flashback scenes also applies to dream scenes. While dream scenes may not be used as often for backstory, they can be used as foreshadowing. Often, though, writers use them as a chance to do...I don't even know. I had a bad habit of writing dream scenes when I wrote Secrets of the Legend Chaser, and I mainly wrote them when I felt like using prose that was a little less down-to-earth (so, basically, when I wanted to pretend I was much more eloquent than I actually was). The problem with these scenes, though, was that they were pointless. So I deleted them all. Unless they're vital to your plot, you might want to consider deleting your dream scenes, as well.
- Detailed physical descriptions of people. Writers love to write descriptions of their characters. They know every detail of how the character looks, so why not share it with their readers? It's important to include a paragraph describing each new character, right? No. In reality, few readers actually care about the small details of a person's appearance. Give me eye color, hair color, body shape, and any other defining features, and I'm good. Any more than that is just a waste of words.
Cassandra Clare somehow gets away with this and I'm not sure why.
- Excessive internalization. Internalization refers to descriptions of the inner thoughts and feelings of characters. Sometimes, internalization is written as a direct thought, possibly in italics. Other times, it isn't so much a direct thought as a description of a character's thoughts, feelings, or state of mind. Internalization is necessary--you can't have a decent book without it. Still, though, there comes a point when enough is enough. We don't need to be privy to your character's every thought. When internalization becomes excessive, it slows down the action and bores the reader.
- Descriptions of a character waking up and subsequent morning routine. Starting a book with a character waking up should be considered a felony, at this point. It's cliché, and annoying. As much as you may think your character's routine is new and different, chances are...it isn't. Not to readers, at least. Like flashbacks, maybe you want to write this description as a way to get to know your character. There's nothing wrong with that, but please, please delete it from the final draft. Your readers will thank you.
- Excessive worldbuilding tours. We know you did your worldbuilding. We know that you spent hours figuring out every detail of your invented setting. I get it. But there's never, ever a need to share every single detail with the reader. I posted more about this here.
- Scenes from the villain's point of view just to show the evilness. Think of those scenes in the Lord of the Rings movies where the shot suddenly flashes to Mordor, showing clips of orcs making swords, monsters being born, etc. Think overwhelming bass drum beats and the Imperial Death March. In some cases, like LotR, these scenes are effective, mostly to create mood and tension. Unfortunately, though, sometimes these scenes from the villain's point of view aren't necessary. They show how evil the bad guy is, but many times, that's about it.
- First-day-at-a-new-school scenes. Ugh. I can't stand these. Like I've said so many times in this post, maybe you want to use these scenes to get to know a character and their life. However, first-day-of-school scenes have become so cliché and overused that they aren't worth it.
- Excessive descriptions of training sessions. This is especially a problem in fantasy books. The hero begins some sort of quest, or starts attending a new school for wizards, warriors, etc. And the training scenes begin. And go on. And on. And on. Some amount of training scenes is okay, if they're necessary to the plot. But there's no reason to go Ender's Game with them (Ender's Game is basically an entire novel of training sessions). Just give readers a Mulan-style montage or something and they'll be fine.
What things are you aware of that writers love to write but that annoy readers? Are there any types of scenes that you know you'll delete but you write them anyway?