The first difference new Mac OS X Lion users will encounter is the inverted page scrolling. Previously you would have to move downward on the trackpad or mouse to move the page up; and move upward to move the page up. Mac OS X Lion by default will invert this behaviour by what Apple called “Natural Scrolling”. Lion has taken a clue from iOS which has the same scrolling behavior.
The difference seems small but it might take awhile for you to get used to. You can change the scrolling direction back to Snow Leopard from System Preferences. But we recommend you keep the Lion default, get used to it and pretty soon it will feel natural to you.