Some additions to make it a killer app...
1) Improved Note taking capabilities: Even if you were to include a notes application like the standard notepad on the Ipad it would improve the user experience. The way it is now, it is hard to type notes and go back to edit them. The stylus function is great, but i prefer to type my notes. The Noterize app is a nice combination of your current notebook and the standard Ipad Notes app which i think is a good compromise.
2) I love the "Send page by email function." It would be good if on the slide bar below, you could check off additional pages to include in email - in case you wanted to include more than one in the email.
3) The interface is very easy to use. However, I now have many folders running across the bottom of the screen. And the two options for document lists are user friendly but both options have large icons. My suggesion would be a third look - which is more of a list system. Folders down the left hand side and list of documents down the right. Very similar to the look of ReaddleDocs. This makes the management of a large number of documents and folders easier.
4) Improved highlighting functionality like Goodreader for pdfs.
5) Bookmark function missing
6) Potential for accessing more than one dropbox or googledocs account without logging off of the one account you have.
This has the potential to be the best app on the market. I just needs some slight alterations.