Seam has been designed from the ground up to eliminate complexity at the architecture and the API level. It enables developers to assemble complex web applications with simple annotated Plain Old Java Objects (POJOs), componentized UI widgets and very little XML. The simplicity of Seam will enable easy integration with the JBoss Enterprise Service Bus (ESB) and Java Business Integration (JBI) in the future
Please note that the Seam developers work very hard to keep several arms-length distance from JBoss. Seam is not about getting you to use JBoss AS. Seam is about enhancing Java EE and its technologies (such as JSF and EJB 3). It also opens the door to many third-party non-JBoss integrations, such as Spring and iText (to mention two).
I would like to leave you with an analogy to help you understand Seam's role. Seam is like a connector as described in "How to Win Friends and Influence People". A connector is the person at a party who introduces you to everyone and finds a way for you to communicate with them (perhaps by suggesting a topic). Seam pulls things together and generally makes for a better party.