When the purists are the advocates, REST is very strict about the resources the verbs affect. So before we had APIs that were "this user places a hold on this book" and i would have thought of URL paths as users/[user id]/holds to see the user's holds or books/[book id]/holds to see the books hold. Instead, one has holds/ and search for the user or search for the book to get the list of holds. So instead of a book object or patron object with hold attributes, holds are now their own resource. That might not have been the best example -- there are other attributes of users that are now off in their own resource because they are managed differently.

Maybe that's obvious to a software engineer, but it's been a mental shift for me.