AJAX
I'm a big fan of AJAX, but imagine if he had a real weapon I think it's used inappropriately in some places. When I'm following a thread and want to see if there are new posts, I expect to be able to hit F5 while it's sorted by most recent or in flat view and see the new posts. But since sorting and view switching are AJAX calls, refreshing the page loses the changes resulting from those calls, so the thread is reloaded in its default view and I have to do extra work to try to find new posts (re-sort by Recent, which doesn't show me replies; re-switch to flat view then navigate to the last page, a difficult task when clicking the "last" button in a long thread only jumps around 15 pages rather than taking one to the "last" page as one would expect; or click the "new" button which from my experience is far from reliable).
In short, when I'm reading page #x in a thread and I refresh the page, I expect to see an updated version of page #x.
(also, when I submitted this I was met with a blank page and going back failed to repopulate the form so I had to re-type it.)