I’ve seen a number of reviews of Safari 4, but I have not seen any cover the importance of promoting history search to the Top Sites page.
Safari has had history search for awhile. To use it in Safari 3, you had to click the Bookmark button, then click History in the list on the left, then click in the search field. Now, history search is right on the Top Sites page, in the lower right corner.
Why is this cool? Because Safari is actually indexing the contents of pages for you. It is not just remembering the titles and URIs, but what was on a page as well. Thus, using history search, you can search for “drum” and if “drum” is part of a comment on a youtube page that talks about “Rock Band” in the tile, it’ll be found!