Some of these are from my own experience but others are from editing or beta reading for others. There are some things that clearly mark a writer as an amateur. Now don't get me wrong these things are a matter of style and styles change so they were often alright in the past but if you want to be picked up by a current editor or catch the modern reader you kind of have to tweak your style. Victorine Lieske once said it was like a Big Mac. People know they can go to any place and get a Big Mac and know what they are getting. If you start changing things like the sauce you are getting further away from the Big Mac and instead end up trying to sell a chicken sandwich to someone wanting a Big Mac. So these rookie mistakes are more about out of fashion styles than real mistakes.
Exposition is the nice term but when people get nasty they'll call it an info dump. This is the scrawl at the start of Star Wars. It is has gone out of fashion mostly because it is a big flag that says the author is a tells rather than shows in their writing.
2. Exotic dialogue tags
When you look at JK Rowlings' Harry Potter people tease her about her various use of dialogue tags. She will never be able to undo those no matter how many millions she makes. Said is completely fine when you are writing about someone talking. They become invisible and most people won't even read them. Put in more interesting ones and you risk slowing down the reader or ending up with a Ron ejaculating about his wand.
Prologues have a place but I can read a prologue and tell you just how many books that person has written because they tend to cram backstory or world building into the prologue. I am guilty of this rookie mistake. It is a signal that the writer will be telling rather than showing.
4. Head hopping
I had already written several books before I even knew what deep pov was. This is a style thing as in the past omniscient POV was acceptable but now it is labelled as head hopping and looked down by readers. Mostly because it creates a distance between the reader and the protagonist.
5. Chit Chat
There is a reason why people on movies never say good bye on the phone. It is too much chit chat. People are used to the day to day language of conversation but when they read a book they want drama. Chit chat drains the drama out of any situation. Avoid it if you can.