Realistically: The ability to show or hide objects on a page. This would make creating things like tooltips, lightboxes, dialogues etc. a lot easier.

Ideally: I would like components to match their actual behaviour more closely, e.g. a tab container would actually contain views in the respective tabs that would be shown or hidden depending on the tab selected. I know this is no easy feat - I have yet to find a wireframing tool that lets you infuse interactivity as easily as one would like to, and I have been through a lot of them.