I was honestly expecting much much more. Yahoo has talent, and YUI was the best library out there when it was new.
YUI was released the same year as jQuery. Care to qualify that a bit? Edit: Had my dates wrong.
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.