Used properly, AJAX can make big improvements to site usability.
There is one issue with loading partial content through AJAX that annoys the heck out of me though, and that's the interaction with the browser's back and forward buttons. When using those buttons, I often notice that the AJAX parts get reloaded from the server over and over. Examples of this include browsing source trees on Github and in Trac.
Given that issue, I do think AJAX could improve voting, and editing previews, and the chatterbox. For loading and rendering nodes, I'm not so sure.