OPDS 1.1 introduces key new features like faceted search/browsing & indirect acquisition. Here's the full changelog:
- support for facets (Spec)
- support for indirect acquisition (Spec #1, Spec #2 & Spec #3)
- recommendations for advanced search (Spec)
- new rel value for recommendations (Spec)
- new media parameter to identify navigation/acquisition feeds (Spec)
- updated Relax NG Schema (Spec)
For a quick introduction to these new features, you can read the following posts: