Here is an interesting interview with three of the wicket committers (or, should i say legends, personal heros ... ) about how Wicket hides the complexity of AJAX-related JavaScript coding from the productive programmer:

Learning JavaScript is not hard, what is hard is learning FireFox JavaScript, IE6 JavaScript, IE7 JavaScript, Opera JavaScript, WebKit JavaScript, etc. The differences are subtle but can cause huge hang ups and are very annoying to Java developers who are used to Java being much more consistent. Another pain point is the lack of a really good JavaScript debugger, which is a tool Java developers take for granted.

and further

Wicket takes care of users having to marshall and unmarshall data between server and browser by working around the problem. Instead of sending data to the client Wicket renders the components on server side and sends the rendered markup. This might not be as efficient, but it sure makes it much easier and faster to develop ajaxified behaviors. For example, if one wants to update a panel that shows user information on the client via Ajax all that is necessary is to tell Wicket that the panel should be updated. There is no marshalling the User object into a JSON representation, and no unmarshalling it on client, neither is there code to manipulate the DOM on the client.