Chris Laffra at IBM developerWorks - Considering Ajax, Part 1: Cut through the hype:

"The major reason for the success of the Web is the predictability and simplicity of its UI model. Basically, anyone can move a mouse, click on a link, move a scrollbar, and hit the Back button. With the growing popularity of Ajax, the risk is very real that developers will go overboard and essentially make everything clickable and change the UI in an unexpected and asynchronous manner. The last thing you want to do is force your user to think.

[...] In this installment of Considering Ajax, I discussed the hype that currently surrounds this technology. You also saw that reliable frameworks are still under construction, and that you should worry about navigation history, bookmarkability, feedback, persistence, concurrency, and security."