Google gave web site owners some tips for optimizing their sites, which were part common sense and part its observations of how Instant Previews preformed during testing (short answer: users are 5% more likely to be satisfied with their search results if they get to previewthe sites).
Across the board users will be treated to improved search results for photos, tweets and accounts, along with a preview section for the Discover tab that takes a peek at the latest trends and activity.