...a unified interface design that is consistent across applications and a control panel or navigation bar (Google Toolbar?) that easily lets me toggle between all my Google apps.
...the possibility to have a local repository for backing up and accessing all my content offline (Google Gears will do this?).
...a unified search that enables both public searches (Web pages, documents, images, people, products, books, blogs, papers etc) and private searches (documents on the desktop, gmail, calendar events, contacts, docs & spreadsheets, notes, blogs) from one single search box.
...tagging of any kind of content item (tags are of course shared accross applications) and an interface where I can easily manage all my tags.
...the contact list with Google Talk as an integrated part of all applications with the possibility to drag-and-drop content items to the contact list to share them with my contacts, and to drag-and-drop content items from a conversation or e-mail to work with them (or store them) in the proper application.
...an explorer interface where all my content items are available (such as the one used for Google Docs & Spreadsheets) and can be browsed and filtered by tags and others metadata such as content type, author, importance and creation date. With versioning of all items and the possibility to share and discuss them with any user I invite.