High level summary is that Ajax, rich internet user interface design, mashups, and user-generated content can create a lot more complexity to the overall user experience than they are worth. Here at Northridge, we try to stay abreast of new tools and techniques, but try to use things judiciously and appropriately. Have you ever tried to hammer a nail with a screwdriver?
No need for me to go into too much detail…read for yourself:
http://www.useit.com/alertbox/web-2.html