My web application contains a list of elements, which can be sorted by drag'n'drop. How should this feature be visually represented to users?
I saw a presentation by Sean Kane from Netflix a few years ago, in which he described how the DVD queue works. You should study it if you can (if you have an account or know someone who does).
A couple of points to note:
He said the default move cursor didn't test very well, so they switched to a grab cursor, as suggested by GoodEnough.
The drag-and-drop is a progressive enhancement. You can still sort by filling in text boxes in the list order column. Many users never notice the drag-and-drop feature is available.
There's no drag handle. You can start the drag with mousedown anywhere in the row (except where another object, such as a link, is in the foreground).
I suggest you use the little gripping hand (open when hovering, closed while dragging). Have some sort of gripping icon (a handle) that looks like something that can be grabbed (in Gmail it's a pair of dot columns (4 dots per column).
I would also suggest that you add a little animation showing the behavior to new users (or existing users if it's a new feature), just don't constantly show the animation every time a user is in your app.
Dragging is a drag. After having recently finished revamping a couple of drag-and-drop UIs, I’ve actually come to the conclusion that next time I’ll change them to a sort of “click and drop”. Have a widget on each item that you click to select it for moving. Then you click where you want it to go. This seems to be going well in testing so far.
Universal Whirlpool Symbol may be?.
The desktop application I work with allows files outside applications can be dragged into. Certain applications which allow drag-n-drop usually have a drop area or content pane; like a playlist in a music player. Drag-n-drop usability is a progressive disclosure but some users will never get to it intuitively. In such cases, The best I could think of advertising the drop-area is by having a whirlpool symbol background.