We've seen fancy PDF documents with animations, lame PDF calculators, but where is the real beef? Where is Web2-like stuff, where are AJAXy PDF eBooks?
The platform appears to be strong enough, the Adobe Acrobat Reader market penetration must be huge, so why smart eBooks still nowhere to be seen? I can imagine lots of opportunities:
Autcomplete search field prepopulated with index words? That would improve search in huge documents, which is still a nightmare despite all Adobe efforts
Dynamic context ads in eBooks? Many would hate it, but authors and publishers would appreciate such revenue stream. Say, small text-based context ad on every 5th page wouldn't harm much. After all unlike Web pages eBooks usually have lots the real content so it must be easy to produce really well targeted context ads.
Social features in eBooks? That might be huge. eBook readers form natural social community, which is currently completely hidden. "Recent readers" sidebar, annotations, ratings, comments, chatting, "Digg this book"?
Autoupdating eBooks? "New book edition is published, get it here" or even "Book updates available, download?". Why not?
These are just few the most obvious ideas. Sure you can come up with more.