I'm looking for solutions that make it possible to create AJAX-enabled web applications without need to write JavaScript code manually. The requirements are:

Performance doesn't matter. It can be slow, JavaScript may be unoptimized, amount of code it generates may be large.
Platform doesn't matter as long as I can work with it on either Linux or Windows.
Language doesn't matter as long as it's Python, Java or C#.
I want to be able to create composite 'widgets' that can be updated dynamically using AJAX. For instance, I want to be able to load data dynamically and replace existing controls with the other ones when user clicks the button on that page.
As long as this solution provides concepts like 'Panel' (composite widget), 'EditBox' and 'Label', I don't care if it's hard to create my own controls.

What do you think?
I'd recommend you GWT from Google. It is great, well done framework that allows creating Web 2.0 applications without dealing with HTML/JavaScript at all.
AJAX web applications with no hand-written JavaScript