From what’s being said by Microsoft Office team, while talking about Office Web, it was stated that Office 14 will have a lot of focus on collaboration for users while working on documents. A demo of OneNote was shown where changes made on the rich version (desktop) were synced across with the Web version of OneNote (Yes, Office Web will have OneNote). Office Web will be cross-browser and cross-platform.
In another demo featuring Word, it was said that users can simultaneously author a document, they will not be over-writing paragraphs and they can communicate with each other from within Word. This was in the desktop version of Word in Office 14.