For WotlK alpha version... the Quest button will not change on use, and only the popups can be clicked on to activate. So whatever is displayed on the bar itself can't be used, or changed.
Edit: I have also found that the trinket buttons suffer the same problem. Also, their position can't be changed within the bar. Pots and whatnot appear to have no change in functionality, but quest items and equipment won't budge.