For me over complicated text. The use of big words to make the writer seem more clever. Please fog test, we are not all rocket scientists.
Another thing I found with a book I just read was the author had a character called Vincent Paul and constantly swapped between the two names.
I did a marine surveying course a few years ago and in the assignments for report writing it states it is the writers responsibility for the reader to be able to understand.
Oh and fluff. Luckily I have great editor whose also not a fan of fluff.