It gobbles up a huge fraction of search results from Google, even things that aren't really image related. Makes finding the original sources for reposted images extremely painful, infuriatingly difficult to avoid without adding -site:pinterest.* to the query, ...
Host a huge number of images that show up in search results and then make them impossible to view without logging in, don't allow right click so you can't save/view image in the normal manner. Basically all the annoying things they can to monetize pictures that people have uploaded.