Web 2.0 Design Elements: rounded corners, gradients, diagonal hatch backgrounds, large colorful icons with reflections and drop shadows, large fonts over 15 pixel, glossy 3-D elements, apparently random highlights and call-outs in text, a supermarket sticker, web floor effect, and beta sign.
Web 2.0 Techniques: Rich Internet Application (AJAX), CSS, Semantically valid XHTML markup/Microformats, Syndication and aggregation of data in RSS/Atom, Weblog, Mashups, REST/XML Webservice APIs.