Updated 2009-06-09: This post is for the Safari 4 beta and will not work with the new Safari 4 released yesterday at the WWDC keynote. I've had a look through that release and can't see any way to revert the address bar, etc sorry.
Having a quick poke through the new Safari binary yields the following strings:
$ strings /Applications/Safari.app/Contents/MacOS/Safari | grep DebugSafari4 DebugSafari4TabBarIsOnTop DebugSafari4IncludeToolbarRedesign DebugSafari4IncludeFancyURLCompletionList DebugSafari4IncludeGoogleSuggest DebugSafari4LoadProgressStyle DebugSafari4IncludeFlowViewInBookmarksView DebugSafari4TopSitesZoomToPageAnimationDimsSnapshot DebugSafari4IncludeTopSites
NB: Run these commands in Terminal.app and then you need to restart Safari for them to take effect.
This moves the tab bar back where you expect it to be:
$ defaults write com.apple.Safari DebugSafari4TabBarIsOnTop -bool NO
DebugSafari4IncludeToolbarRedesign and DebugSafari4LoadProgressStyle
When both set to NO it restores the blue loading bar behind the URL. Also puts a page loading spinner in the tab itself, which looks odd with the new tabs.
$ defaults write com.apple.Safari DebugSafari4IncludeToolbarRedesign -bool NO $ defaults write com.apple.Safari DebugSafari4LoadProgressStyle -bool NO
Switches off the new URL autocomplete menu and goes back to the original one.
$ defaults write com.apple.Safari DebugSafari4IncludeFancyURLCompletionList -bool NO
Turns off the new Google suggest menu.
$ defaults write com.apple.Safari DebugSafari4IncludeGoogleSuggest -bool NO
Removes CoverFlow from the Bookmarks view entirely. (Credit to Erik)
$ defaults write com.apple.Safari DebugSafari4IncludeFlowViewInBookmarksView -bool NO
Disables the dimming when you click on a Top Site and it scales the screenshot up to fill the screen.
$ defaults write com.apple.Safari DebugSafari4TopSitesZoomToPageAnimationDimsSnapshot -bool NO
Disables Top Sites feature completely.
$ defaults write com.apple.Safari DebugSafari4IncludeTopSites -bool NO
Just run the defaults command with the
delete flag for the appropriate key you wish to delete.
$ defaults delete com.apple.Safari <key>
NB: Don't include the
-bool NO at the end, it just requires the key (eg: "DebugSafari4IncludeGoogleSuggest")
Jools points out in the comments how to reset the recent searches in the google search box.
Lowell's kindly created a Mac OS X application to edit these settings without using Terminal. http://github.com/cocoastep/tweaky
Patric has kindly translated this post into Belorussian and posted it on his site.