Well, it's not officially a missing function according to the W3C spec, but it is a function that I have found very useful over the past couple of years of DOM work.  The missing useful function is, as I call it, cancelEvent.  According to the level 2 spec, there are two main functions that can be called on an event (Level 3 has more), preventDefault and stopPropagation.  preventDefault prevents the default action of the event, for the element the handler is registered on.  stopPropagation keeps the event from propagating (bubbling).  cancelEvent is a combination of the two, i.e.:

function cancelEvent( event ) {

Where is this useful? Anywhere you want to prevent the default action from happening and you want to stop the event from bubbling. ;) DOH. My particular use has been buttons of all sorts where I do not want a link to open, nor do I want any click handlers further up the DOM being called.

My patch to jQuery (currently working off of version 1.6.3):
find the line:

        isImmediatePropagationStopped: returnFalse

and overwrite that with:

	isImmediatePropagationStopped: returnFalse,
	cancelEvent: function(){
		this.preventDefault(); this.stopPropagation();

To use in your event handlers that take the event as an input parameter:

function foo( event ) {