I have a pet peeve about people trying to write Sherlock Holmes mysteries. There are a few authors who can manage it - Laurie King, for example - but most of the time it just doesn't work. I picked up one such book, and got rid of it after five pages because I'd started cataloging all the ways that Holmes and Watson were misrepresented.
I agree with the PP who said dialectical spelling. If I have to spend half an hour reading a single page because I can't figure out what's being said, then it's too hard. It may be authentic and genuine, but I don't think it makes for a better book.
I hate when an author suddenly starts to include fantasy or gothic romance in the book. I'm good at suspending my disbelief, but if the first half of the book is a "normal" mystery, romance, whatever, and suddenly the main character hits their head, wakes up and is seeing purple unicorns or sexy fairies, it's very jarring and hard to work with.