1. Yahoo hosted scripts, including roll-ups
2. UI components and styles
3. The best documented library, both outside and inside thde code, hands down
4. More feature complete (ex. one I can recall off the top of my head, form handling of upload inputs didn't exist in jquery, and extending jquery internals was/is a nightmare in comparison)
5. The best documentation, hands down
6. Yahoo hosted scripts, including roll-ups
7. The best documentation
jquery has come a long way with documentation, but then YUI has evolved even further in becoming a modular library.
