The following list, which is in no particular order, contains a wide range of behaviors that an author shouldn't have to face in my opinion. Some are downright hateful (Internet Trolls, Goodreads Bullies) and others are just rude or unprofessional.
What do you think of these? Do you have any to add?
Don't carpet bomb a group of authors. Don't go on a rampage, giving a large group of authors 1 star reviews. That's being a troll, and you are evil if you do such things.
Don't rate a book before reading it. Though a common practice on Goodreads, I do not do it and where DeeJay or I represent the Crew, we don't do it either. Now, some bloggers/reviewers will not review a book unless they have read the entire thing. I don't go that far. I don't mind reviewing a book that I couldn't finish, but I'll make an earnest effort to get as far as I can. If I can't finish the book, I'll give you detailed reasons why not.
Don't bad mouth authors who speak up about a review. If an author responds negatively to a review, I suggest a careful response if you choose to respond at all. Defend yourself where necessary, but don't needlessly say bad things about an author. If you can't discuss things with an author in a respectful tone, then don't engage that author at all. If the author is being a punk, then defend yourself. Rise above the occasion. Don't lower yourself to it.
Don't encourage others to leave negative reviews for authors you don't like. Warning people to stay away from a book because of how bad it is ... that's one thing, but don't campaign to get that book a bunch of 1 star reviews. That's being a bully.
I campaigned once to vote down a review, but I was questioning the value of the review, not the book itself. And definitely not the author. In other words, I was exposing bad behavior, not spreading it.
Don't threaten authors with physical violence. We've mentioned Carrie's experience before, and although she's not an author, it's worth mentioning again. Telling someone how to kill themselves or just alluding to it is not right. Other trolls have gone even further, posting a physical address. Why would someone do this?
Don't publicly degrade authors on social media. Though similar to the point above about not bad mouthing an author, this one is about you being the one to start the shouting match. Don't!
Don't bait authors into saying something incriminating. Once again don't be a bully. Twisting someone's words that you put into their mouth is childish at best and usually downright mean.
Don't spread lies about authors. Don't be a bully. Gossip doesn't help anyone. And it's mean. Don't do it.
Don't spread negative truths about an author. You may know that an author picks his nose, but it's mean to make that public.
Don't write a negative review simply because of bad formatting/technical issue. I'm not talking about bad grammar or some other legit issue. If you can't read it because all the paragraphs are bunched together, for example, send the author a note to tell them about your issue and ask for an updated copy. Don't be a punk and leave a one star review. Of all the things listed here, this one is probably the one that some reasonable people may not find fault with, but I think it's highly unprofessional.
Don't write a negative review for something that isn't the fault of the author or direct agents of the author. For example, if there's a glitch on the retailer's side. "Couldn't download it," says many a 1 star review. Not the author's fault.