Making a DOM element draggable isn't terribly difficult - there are jQuery plugins and even some native HTML5 capabilities in newer browsers. But generic dragging is kinda boring. What about smooth, natural momentum-based continuation after the user releases the mouse (or touch on mobile devices)? What about imposing bounds and edge resistance and silky-smooth redirection to certain landing values that you provide? Or flick/drag scrolling with bounce-back that doesn't look awkward? Or instead of dragging to change the position of an element, what if you want to spin it? Maybe you even want to track the momentum and let it come to rest naturally or rig it to glide to a stop at a certain value, Wheel-of-Fortune style? Yeah, that's a lot more complex. In fact, it can be a real drag to build (sorry, the pun was irresistible).