I'll state for the record that the new address bar functionality in Safari 5 is the most un-Apple interaction I've seen come out of Cupertino in a long time. Perhaps adding RAM to my 8500 was more painful, but when a company known for ease-of-use screws up something so basic, it makes you wonder, "who thought this was a good idea?" But Safari 5 is "smart." As in, "I'm smarter than you, nya nya, and you'll go to the sites I say, not what you want!" It goes beyond preferring your history to primary domains. It'll basically get in your way and make a constant mess of things. There's a MacRumors forum thread detailing some specific bad behavior.
So here's the thing. In Firefox, you can type y-a-h-o-o, hit Return and the browser is smart enough to presume you meant yahoo.com, adding the .com and whisking you away to the search portal's main page. Safari 5, however, will not give you the top-level domain unless you frequent the site regularly or bookmark it -- even then it won't necessarily pop up top. Instead, it'll presume you meant to search your history for all the instances of "yahoo," and that includes titles of pages. So a post about "some yahoo" on a random site you checked out yesterday trumps the top-level domain, or possibly a page titled "Yah, I love pizza!" that you visited the week before. Even in ancient versions of Internet Explorer you could hit Ctrl-Enter to append .com (and prefix www. for you old-timers) and it worked great.
One particularly goofy behavior is redirection. For example (and as noted in the MacRumors thread), if you do a Google search for 'ebaumsworld' and click through to the site via Google, the next time your genius Safari browser sees you typing 'ebaumsworld' in the address bar it will not finish typing ebaumsworld.com, no no. That's apparently dumb. Clearly you meant to access the Google search redirect URL, which is a mass of gibberish to the average person and serves no purpose once you've found the site you were looking for. Why?