Web 2.0/AJAX is nothing new, but there are more and more companies or web developers putting these buzzwords on their web site without any knowledge of these techniques.
You should watch out for updates of your web application. If you are using AJAX there may be requests during your update that may fail. Use error handling where you can. Have a deep look inside your code (and the source code of your used frameworks) if interfaces will change. A new version can stop old opened web pages running.
More and more web sites using AJAX to completly update the page without a postback. If you press F5 (reload) you will be on a different page. Yes, I know there are some frameworks/libraries that are working correct. If I want to open the "Contact Us" link in a new window (Shift + Click) it will fail, too. Have a deep look on your log files and track visitor ways.