The ASP.NET AJAX Framework is an embarrassing server-side centric approach to DHTML / AJAX web development. While most programming languages and frameworks come with both good and bad parts, the ASP.NET AJAX Framework is probably an example of an overall bad part of ASP.NET - on the contrast the ASP.NET MVC Framework looks to be a good part.

What's wrong with the ASP.NET AJAX Framework?

1 .NET Developers are DUMMIES!
The ASP.NET AJAX Framework appears to have been designed under the assumption that .NET developers are dummies and can't learn or don't want to learn JavaScript. That .NET Developers would rather hobble along with their familiar languages, then to learn something new. I understand that the ASP.NET community's only real problem is education, so let's ask: What is wrong with the ASP.NET Community? Then educate ourselves rather than becoming the .NET Developer statuesque. It's patronizing to use a framework that assumes learning a new language is beyond our capabilities. Many of these other programming languages also happen to be more expressive than the statically typed .NET languages.

2. The "don't write a line of JavaScript" abstraction leaks like a sieve
The Framework is intended to shelter .NET Developers from the big bad JavaScript language. Which, like driving a car across North America without knowing how to pump gas, stops you dead. Either you depend on someone to pump your gas - depend on something like the ASP.NET AJAX Control Toolkit and the many authors to write your JavaScript - or you stop moving. As Web Developers, sooner or later learning how to pump your own JavaScript becomes a mandatory skill.

3. Client-side programming from the Server-side is a absurd
The AJAX Framework does back flips to translate server-side code into JavaScript, and then requires that you write JavaScript anyway. Save yourself the pain, learn JavaScript. One day The Law of Leaky Abstractions comes into play, the gas station attendant fills your gas tank with diesel - your AJAX Control Toolkit blows up (requires debugging) so you have to learn JavaScript anyways.

4. Bloated, poor performance, bad user and developer experience
The AJAX Framework extends many of the native JavaScript objects as it attempts to turn JavaScript into a staticly typed programming language, and tries to hook into the ASP.NET life cycle, but all these features are unneeded as they are ALL already achievable through the native JavaScript language - if it walks like a duck and quacks like a duck, then... err... could someone remind me why we need typed languages in a web browser? Anyhow; all these object extensions, enhancements, and upgrades, contribute to more scripts that need to be downloaded and increases the number of scripts running in your browser. Then to make matters worse there's partial-page rendering and Update Panels which do full page post backs under the guise of AJAX -bad-bad! Client-side scripting is supposed to enhance the user experience not make it worse. Other AJAX Frameworks are built with performance as their number one goal, but in the ASP.NET AJAX Framework. Adding more widgets to JavaScript seemed to be the first priority, and performance an after thought.

5. Working against the grain is a waste of time
The AJAX Framework works against the grain, it would be nice if it embraced the JavaScript language. Very few of the concepts and metaphors used in the ASP.NET AJAX Framework transcend AJAX techniques or frameworks - your time is probably better spent learning how JavaScript works or how the other AJAX frameworks work.

6. Disconnected from the ASP.NET MVC Framework
The ASP.NET MVC Framework throws the ASP.NET life cycle away leaving more dead weight ASP.NET AJAX Framework script and rendering many of the AJAX Framework techniques moot.

7. The ASP.NET AJAX Framework almost over looks the 'J' in AJAX - 'J' stands for JavaScript, and that is GOOD
JavaScript is the glue of the web, even the ASP.NET Framework depends heavily on JavaScript, it is not something to shy away from.

8. Aside from local intranet sites, no one really uses the ASP.NET AJAX Framework.
The AJAX Framework isn't widely used. The ASP.NET AJAX site showcases 25 sites using ASP.NET AJAX. None of these applications appear to be high-traffic or moderately high-traffic applications. On the other hand, the YUI site showcases 89 sites, out of these sites, 6 sites (flickr, Slashdot, Linkedin, Paypal, O'Reilly, My Opera) could be considered high-traffic. Other AJAX libraries like jQuery, and Dojo compare similarly. Your time might be better spent learning one of the other AJAX frameworks.

If we (.NET Developers) are going to claim we know AJAX, then let's focus on the core of AJAX (JavaScript) and stop obscuring it in poor frameworks. Frankly the ASP.NET AJAX Framework is embarrassing, the web development community is laughing at the ASP.NET AJAX Framework and the Developers touting it.