I think the phrase "Web 2.0" is for the most part universally despised, and I wholeheartedly subscribe to this point of view. While it may be fluffy market speak it does carry a meaning to me, a vague and over hyped meaning, but a meaning none the less. When I work on client sites I will suggest using AJAX or a more interactive design if doing so is a good fit for the site and overall functionality. More often than not however clients will ask for dynamic parts of a site, even going so far as to say "I want this page to be more AJAX-y". Over the last few years I have been involved with and written quite a few different dynamic and interactive web site features. Thanks to great packages like Sajax, Prototype, and Script.aculo.us among others, adding varied levels of web 2.0 goodness while keeping an eye on overhead and bloat is not too difficult.