@Jeruba Well, @Gundark covered a lot of it. It’s nice to have a place to catalog the books you’ve read, own, want to read, want to buy, etc. They have an automatic recommendation feature, as well as a member recommendation feature (where members who liked a book suggest other books like it). They even have a neat Unsuggester feature, which suggests books you are least likely to have in your library, but might still enjoy. All of these features can recommend books off of a single book that you enter or based off of your entire library.
There are also a ton of social features connected to member libraries. LT can show you other libraries that are most similar to yours, people who have added the same books you have to their libraries, people who have the same favorite authors as yours, etc. You can also compare your library to Legacy Libraries, which are the libraries of famous dead people, like Thomas Jefferson and Tupac Shakur. You can also look at global and local statistics, like the number of readers who have a certain book, or how many books you have that were written by female authors. Or dead authors. Or the number of books you have in different languages. You can see clouds that show you the tags you use most often or the author whose books you have the most of. There’s a feature called Common Knowledge where users fill in tons of info about books, like characters, places, first sentence, etc. In short, you have access to tons of data about books.
One great thing they do is give you access to tons of libraries. Most book cataloging websites only access Amazon. If you have a rare book or a rare edition of a book, you’re out of luck. LT accesses Amazon, but also the LIbrary of Congress and 690 sources from around the world. Plus you can manually enter books. Also, most book cataloging websites severely limit your options for exporting your data or importing from other places. LT gives full access both ways.
Then there are the Groups, which are basically book groups. They come in every flavor, from Science Fiction Fans to Twilight Fans OMGWTFBBQ!!!1!! to I Survive the Great Vowel Shift. If you have a few weeks with nothing to do, there are plenty of discussions to browse through. You can also enter location information and select your favorite libraries and bookstores in your area, with LT letting you know about any events like author readings taking place nearby.
My favorite parts of LT are the book reviews and the Early Reviewers Program, which lets you sign up for a lottery each month to get Advanced Review Copies of books (I’ve won at least 3 books myself).
Basically, LT is for socializing, seeing statistics and finding data about your books. This sets LT aside from other sites, which don’t have 10% of the features LibraryThing does. Another great thing is that the creators and programmers of the site are always on the site, answering questions and joining in on the discussion. They even ask members to actively participate in discussions about what new features to put in and how to go about implementing them. I’ve never been to another site that let me feel like my opinion was actually considered and used.
Phew! Longest commercial for Librarything.com ever!