What is LibraryThing?
LibraryThing is an online service to help people catalog their books easily. You can access your catalog from anywhere—even on your mobile phone. Because everyone catalogs together, LibraryThing also connects people with the same books, comes up with suggestions for what to read next, and so forth.
That is the official description from the web site's FAQ.
I really don't see the need to catalogue all my books for everyone else to comment on, etc... Also I can get recommended reading list from any bookseller or library web site along with a description/synopsis. But with that said, I can see how this would be useful to some people and the group discussion at http://www.librarything.com/groups/librarianswholibrar is great. I especially enjoyed the posts about librarian stereotypes in movies such as "It's a Wonderful Life".