"Secondly, the browser isn't a good listener to external events. If you want to build an application, for example, to show you instantly when someone bids or a price changes, it is hard. You can poll, but poll too frequently and the application starts to feel sluggish and it isn't easy to do this. What you really want is an event driven model where in addition to the events like typing the page can describe events like an XMPP message or a VOIP request or a data-changed post for an ATOM feed."
I think this is a big one from a system administrators perspective. Consider the amount of traffic generated by Ajax apps that are set to pole the server every second or so. These apps will just eat up bandwidth like crazy. We already dealt with this problem occuring with RSS, will the same thing work for Ajax apps?
To implement the event driven model, you would need standards. Then you can blame the browsers for not being standards compliant when an Ajax app that is event driven brakes. Without standards, we are probably going to have each browser having its own event driven model. Ajax (XMLHttpRequest rather) should be standardized and extended.