While I haven't actually watched the WWDC keynote video I did watch a live blog and I have to admit that at the time Passbook didn't seem that big a deal... but having some time to think and then seeing a demo of it by one of my colleagues today, it suddenly clicks in to place.
Essentially, the next few years will see a distinct rise in mobile payments. All those store cards you've got; Tesco, Starbucks and so on, will disappear to be replaced by cards on your phone. In the case of those two examples they already have native iPhone apps to replace your physical card with a virtual one but so far the move has not been ubiquitous probably due to the lack of a unified secure platform with consistent user experience, which Apple now provides in the form of Passbook.
The main focus of Passbook just now is on ticketing. This allows paper boarding passes, cinema tickets, and so on to be replaced with a virtual version. Even better, the tickets can become 'relevant' at specified times and places, popping up on your phone. For instance, you might pre-book to see the latest blockbuster film. You end up with a ticket in your Passbook (probably after downloading it from the cinema's website, or opening an email with it attached) and then when you arrive at the cinema it automatically pops up ready to be scanned.
However, ticketing is just a small piece of the puzzle. With NFC on the horizon, it won't be long before you're paying for all kinds of things with your mobile phone, from interacting with vending machines, to buying a washing machine. Passbook is another step in that direction and in my humble opinion vastly understated.