I'd say the interface is not one of the strong points, since it confuses a lot of people and turns them away from the site. But, it is powerful if you want to engage with it.

For example, I could compare my library to yours and get a list of the books we share, that we both liked, but aren't generally very popular. Or I could get a list of books I've read, sorted by their Lexile score. Or get a list of all the epigraph quotes printed in the books I've read.

Basically, it's a database approach rather than a social approach. You can do all the above stuff on the website, but their API is also really powerful, and free. In fact the whole site is completely free.

Another thing I really like is the Member Recommendations. They have an algorithm that recommends books you might like, based on other books. But, in addition to that, they have a list of books recommended by actual people who have read both books, explaining what it is about book B that might interest you if you liked book A. So, more like what an actual librarian would do. Here's an example of what I mean:

https://www.librarything.com/work/1472#memberrecs

Lastly, I just think the vibe at Librarything is better than Goodreads. They're a tiny company made up of librarians and developers, and you feel the scrappy Web 1.0 charm from them and the community there in a way I never saw with Amazon-owned Goodreads.