Ajax is a useful tool when used wisely. The authors of WhatWhat Status were able to ensure that everyone using the product would have a modern Ajax-capable browser, so they made extensive use of Ajax in the project's page to reduce the time their users wait for pages to reload and to make it easy to view or hide individual page elements dynamically according to what the user needs to see at the time.
The WhatWhat Status authors make use of the widely implemented innerHTML method to update various elements in the page. Even though innerHTML is not part of the standard, it is implemented on every modern browser, and it can be a lot easier to use than DOM manipulations.
MochiKit adds the idea of callbacks to its request objects. You can use the addCallback() method to indicate what function should be run by the browser when a response is received back from the server.
TurboGears includes simple_json that can automatically transform most native Python data types into JSON strings, so it is extraordinarily easy to create methods that return JSON to the browser.