When you tap into the Safari's URL bar in iOS 4 and start typing, Safari starts to do a "keyword search", i.e. display predictive results based on your bookmarks and history. Anything that contains the text you're inputing either in the URL or history is listed below the URL field so the moment you see what you want you can just tap it and go.
This makes it easier to find something if you don't remember the exact web page address or if you know you recently saw a site, and know what it was about, but don't remember where exactly it was. Just start typing a few words you do remember and let Safari do the heavy lifting. Highly convenient and certainly "awesome". [To misappropriate the term from Mozilla]
When you try out the new Safari URL bar, let us know how it works for you. For more, see our complete iOS 4 feature walkthrough.
[Thanks to @jaerre for the reminder]