I'm so guilty of the lyrics in the middle of random stories to set the tone LMAO. I did that once a few years ago.
Now if I do it, it's either because it's a song fic, or cause someone's singing it. I give the song a purpose lol.
Fanfic pet peeves...the author's name being the lead of the main character! At least TRY to be subtle! If you put your name as the main character, you're basically waving a banner announcing "THIS STORY IS NOTHING BUT MY FANTASY. ENTER MARY SUE FIC AT YOUR OWN RISK!" <-- I mean I did that at one point, when I was 15 LOL. No excuse when you're an adult.
Another pet peeve, self inserts into the banner. It's a more shameless example of the previous one. I mean for the love of all that is holy, to me, I feel like shaking the person. I don't want to read your fantasy, so thank you for making it VERY clear that this is what that is.
Dialogue being too wordy. And by this, I mean dialogue that doesn't flow. For each bit spoken, there's a paragraph of backstory or description attached. Now, this isn't a sign of bad writing. Hell, I've seen very well written stories guilty of this. It's just for my own personal liking, dialogue should flow quickly, it's supposed to move everything along. Personal pet peeve, cause it makes me skim things lol.
Descriptions of a character every single scene, down to the change of clothing. No. We don't need to know. We just don't. It's not that important. Vividness in description is awesome, but it goes overboard.
Overly flowery writing, this goes with one of Julies. Over the top descriptions that are written with obscure colors or synonyms for common words. Just for the purpose of sounding pretty but actually describing very little.
Convenient plot twists. If you write yourself into a corner, don't try the easy way. (I'm looking at you HP script writers and your "Horcrux sensing Harry!". If you'd stuck some backstory in, you wouldn't have had that issue!) Work your way out. Don't just toss something in, research every option and realistically write your way out of that corner.