It's actually pretty nifty. You can collapse your reply to a bar at the top of the page while scrolling through the rest of the thread. And some things that weren't available via buttons are now (e.g. the float tags). And if you have a reply open, hitting quote on different posts adds them to your reply with proper attribution.
One usability difference is that (at least on other forums) quote gives you a text-area within the HTML or something and hence if you somehow happen to lose the tab you can normally bring it back WITH content using the browsers undo (I think some browsers do undo with java-script to now but maybe it's hard with something like quick-reply? Easy enough to find out whatever both or either work.)
Which is good when bad things happen.
With AJAX and other java-script crap you usually (didn't) have that luck and whatever data you'd put into the element is gone forever.