We are no longer covering this topic in the book. You'd probably get better answers on the Seam or Spring forums, but this is a pretty broad question.
One interesting thing that Seam has is conversation management. I feel like this is going to emerge to be one of the most important concepts in application control both on the server side and eventually even on the client side with RIA frameworks. I haven't done my DD, but I haven't heard of any other Web frameworks that support a conversation scope/context.
I would second that.. playing with Seam, conversational state management seems to be the fundamental 'out-of-the-box' thinking that excites. On the surface of it, it seems to be contrary to some J2EEpattern thinking. But I'm inclined to think it's right. Also Seam leverages JSF and IMHO takes it to a new level. Though I'd like to see more on how scalability really stacks up.