@eschwartz: In (very) short:
From a user's POV: This is NOT an epub reader. The ePub renderer is horribly crippled compared to their own kepub renderer. Which of course leads to fun times w/ a cross-platform library, since the kepub format is not as well reversed as KF8.
From a tinkerer's POV: The system might be 'open', but it's not fun at all, and Kobo's software is a brick-wall in terms of possible interaction with it.
I'll probably go into more details (and in less black/white terms) at some point
.