Since Kindle books seem to be slightly more expensive than paperbacks (but noticeably cheaper than hardcovers), I'm guessing that's not it.
Guesses (since I'd probably buy one for $200, but not $400)
- "digital paper"-style eBooks (like the Kindle and Sony's eBook reader) are a pretty serious improvement over standard LCDs for books
- the ability to download books right away, from almost anywhere is pretty cool, and you don't have to subscribe to anything to use it