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saying that a book stinks will be viewed by some people who like the book as a criticism of them, even if you do not directly call them idiots

Then they are oversensitive, IMO. Art is art. We all have personal tastes. When we dislike a piece of art, our dislike does not make any automatic statements about people who do like it.

I mean, I hate turnips. Am I therefore implicitly criticizing people who like them? Of course not.

BTW, I do think "This book stinks" is a different statement than "I hated this book."