Agreed - However, I'm still weary of the internal OneNote Format - I haven't poked around, but last time I checked, it was a big binary undocumented blob basically - so the lack of a proper structured export is somewhat worrisome.
The UI looks great though - if only office for mac proper could follow with the same quality, it would be great.
I'm actually working on an org-mode onenote sync tool right now and the files are stored in an xml format of some kind. With very little knowldge of the format I was easily able to parse out all todo lists from the blob (which is a zip file btw).
Have a look in ~Library/Containers/com.microsoft.onenote.mac/Data/Library/Application Support/Microsoft User Data/OneNote/15.0/OneNoteOfflineCache_Files
Any sandboxed app has its data under ~/Library/Containers/<app>, then from that usually under the Data/Library folder, which is a kind of mirror of your user library folder, just holding that stuff the sandboxed app is allowed to see.
All I see locally is a ".onecache" file - a binary file that appears to contain the notes themselves, and a directory named "OneNoteOfflineCache_Files" of attachments (embedded images, etc.) I don't recognize the header on the onecache file, so I'm guessing it's proprietary.
The UI looks great though - if only office for mac proper could follow with the same quality, it would be great.