I could see Amazon using the card metaphor as a custom multitasking Android launcher with full power of a pending patent application behind it while ditching the remainder of the OS. It's the one distinction webOS has had in UX and one that's generally garnered the most positive attention.
I could not, however, see Amazon hedging on Android by holding webOS in state. That would be a rather insane move given the audience the Kindle Fire will attract from Day One.
What about Dalvik and the on-going Oracle case? webOS has no Java, there is another distinction for you. It's the full stack in webOS that is unique, not just the UI.
That's partially true; webOS 3.0 has no Java as far as I'm aware, deferring to node.js for its services instead. webOS prior to that (e.g., Mojo era) contains an absolute trove of Java all over the damned place. I remember dissecting the media system looking for private APIs in parallel with developing a webOS application and many points led right back to Java-based services that assisted in doing a lot of perfunctory tasks.
In fact, I distinctly remember asking the Palm webOS developers when I discovered in their Java-based media services a particular call that instantly parsed SHOUTcast/Icecast-style playlists of many various formats with ease whether it'd become a public API call. It never did.
If HP races to finally reconcile the two sides as a promised a half-year ago a la Android's ICS, then there might be no obstacles to the Java argument. Until then, however...
I could not, however, see Amazon hedging on Android by holding webOS in state. That would be a rather insane move given the audience the Kindle Fire will attract from Day One.