Why is it called WebSphere XD? Why not WAS
I even said, "[WebSphere XD] is a J2EE app server unlike any other on the market..." Well, it's not exactly a J2EE app server.
WebSphere XD is not so much an application server as it is an environment for running application servers. It doesn't run the applications, it runs and manages the application servers that run the applications. So far, the app server it manages is one or more WAS ND (Network Deployment) server clusters. You can think of this as XD being built on top of ND, but really XD manages ND, and therefore whatever's running inside of ND.
What makes this distinction important is that it means that WebSphere XD should eventually be able to support other J2EE-based WebSphere products like WebSphere Portal Server, WebSphere Business Integration Server Foundation, WebSphere Commerce Server--anything built on top of WAS ND.
WebSphere XD isn't just a new product in the WebSphere product line, it's one that over time will probably impact much of the rest of the product line, much like WebSphere Application Server itself.