One of the first things that people notice when upgrading to Mac OS X Lion is that scrolling works the opposite way that it used to worked.
Without a doubt, this will seem backwards – at first. Most people have said that it takes several days to get used to the new format. But, why change a paradigm that's work for the last quarter century?
The answer is simple and most people haven't really noticed that this is the exact same way that scrolling works on the iPhone. Before Lion, if you wanted to scroll down a web page, you'd have to grab the scroll bar and pull it down. The metaphor was that you were pulling the window down the page to view the content "below the fold."
Now, the metaphor has been changed to match the iPhone and a layer of abstraction has been removed. Instead of pulling a window down to view the content, you now pull the content up to see it. No more window model – it's just you and your content.
Give it several days to a week to see if you get used to it – it's like learning to drive on the other side of the road. If you use the iPhone, then you'll probably get used to it and not notice it in the same way that you never noticed that the iPhone worked the opposite way.
But, if you really can't stand it, you can change the settings back to always display scroll bars in the General part of System Preferences.