Should bloggers be critical? It's an interesting question. I suppose it depends on who you feel reviews are for. While I'm happy to share positive reviews with authors, I think reviews are for readers not writers. I know of authors who say they've learned from reviews and can see they can be useful but I'd say beta readers, critique partners, agents and editors were better sources of feedback.
I also think authors can overestimate the influence of a single review, positive or negative. For me, buying a book is more often down to word of mouth, especially repeatedly hearing about it on Twitter, than reading one review. Negative and critical reviews tend to get shared far less than more positive ones.
I believe there's room for all sorts of bloggers; that has to include people who'll speak up about problems in books, particularly problematic portrayals of sensitive issues or minority characters. (I think reviewers talking about issues with a book, especially characters from a similar background to them, can be hugely valuable and would hope that the publisher would take note and try to see why they found them to be problematic.)
I no longer write critical reviews - I've had bad experiences with a couple of authors, and have seen far worse happen to some other bloggers who've pointed out problematic content and been stalked, and had their places of work contacted by authors.
When I was more willing to write them, I'd try to be polite and calm in dealing with problems I had with a book - but having said that, I'm aware I'm speaking from a privileged position. As a white, able-bodied, cisgender middle-class teen, the vast majority of characters I read about were similar to most people I knew. If I read a problematic portrayal, there were hundreds of others to balance it out. I can only try to imagine how hurtful it must be to read a portrayal building on negative stereotypes of someone like you knowing that there were few others to contrast it with.
So for me, bloggers NEED to be able to speak their thoughts freely, particularly about books with difficult issues or diversity as a central topic.
Jim Dean is a YA/MG book blogger at yayeahyeah.com.