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?
The only way to "fix" this behavior is to a) constantly delete your history or b) always type the full domain name of a site without searching. Well, there's a third way, which kills your history each time you quit Safari, but again, that's dumb. Even then you may get mixed results, honestly. Safari 5 seems to check how frequently you visit a site, so since I frequent a site called "TUAW" which has "apple weblog" in there somewhere, I have to slide down over half a dozen History-based links before I get to my bookmarked Apple.com link. Again, why? Wouldn't it be most logical to presume you're typing a valid address first, then pull from bookmarks and then pull from your history? No, stupid, Apple is way smarter than you, so just shut up and enjoy the repetitive stress injury you're getting from swiping on your iPad, typing on your iPhone, and vainly trying to visit a very simple URL.
As I said, Apple is renowned for making things that "just work" and simplifying complex processes such as editing video.* Why in the world they thought this was "smarter" than understanding a simple URL, we will likely never know.
*I raise a Golden Fist of Fury to the memory of iMovie '08, possibly the crappiest software to come from Cupertino since AppleWorks.
UPDATE: Since some people aren't believing me, I've added a screenshot of what happens when I try to type "apple" in the address bar. In Safari 4, and every other browser, the top result is apple.com. With Safari 5, it's "apple weblog." This makes sense for some people, but it would be nice to disable it.