Task 1: Tell us: What is the mother of all writerly sins in your book (tropes, grammar mistakes, telling instead of showing, etc.)?
I'm not sure if this is in the spirit of this task, but my #1 writerly sin would be badly behaving authors. Authors who attack readers--either passively or actively--automatically guarantee that I will never read their books.
As far as actual book content, telling without showing drives me nuts. I read a book where most of the action (fights, battles, etc) happened off the page and was later told to the reader my the character narrating the book, and it was so frustrating.