AJAX Architectures Condensed
One thing is using AJAX to dynamically refresh a small piece of a single page; all another thing is designing a whole presentation layer to be partially refreshed in every possible operation against the server. An individual feature can be happily and nicely coded using a smart piece of JavaScript; a whole Web presentation layer will cost you a lot more if done entirely in JavaScript. And from scratch.
Sure, new productivity tools are created every day (from the popular jQuery library to the upcoming ASP.NET AJAX 4.0 framework), but the most effective way of adding AJAX to applications continues to be the subject of research and begins to look like the Holy Grail of Web software.
I just published an article on DotNetSlackers, that summarizes the options you have when it comes to AJAX architectures. It is an excerpt from Chapter 3 of my latest Web architecture book mentioned in this recent post of mine.