Spoilers have always annoyed me. Both when reading and when writing reviews of books, the line between knowing what a book is about and knowing enough to not even need to read the book can be rather thin. This happens quite a bit when you tag things. The main categorization of a book is usually safe. Tagging Spin by Robert Charles Wilson science fiction is in no way a spoiler, but tagging it uplift probably is. It's a perfect tag, but that aspect of the story appears rather late in the timeline and might thus be considered a spoiler.
I can think of other books where tagging them science fiction instead of fantasy could be considered spoilers. The original Pern trilogy comes to mind. There are plenty of examples where the question of science vs magic or alien vs religion is the central mystery.
It strikes me that one benefit of the TagShadow method of recommendation (browsing through clusters of similar books) to an extent protects the reader from the potential spoilers contained in a tag cloud about the book.