More clues to the upcoming Google Drive can be found in the Docs source code. One interesting possibility, that seems likely, is support for third-party apps. Granted, there already is an API for Google Docs, which will become Drive, but an SDK Google is working on will enable third-party apps to integrate into Docs/Drive.
The idea seems to be to enable users to open and edit files not supported by Docs in other apps. Google already has a marketplace for third-party applications and services for Google Apps.
Google Drive will also sport some integration with Gmail, it seems, perhaps the ability to upload a file straight to Drive from Gmail instead of adding it as an attachment.
"Say goodbye [to] email attachments and hello to real time collaboration. Drag anything shared with you to My Drive for easy access," a message in the source code reads.