One of the most powerful features I’ve experienced so far is the desktop search.
From the start button (can’t stop calling it that for now!) the easiest way to find anything be it an application, document or email is from the search bar. For information workers like myself who prefer flatter file structures search is the most important feature an OS can offer.
The type ahead search results are returned quickly within 1 second I have a list of application names for the letters “ex” and within 5 seconds I have all the files and emails/rss documents from Outlook. This will be very powereful especially if vendors plug into this capability to return results for their applications and web services.