Excellent article on an often-overlooked subject. The hand cursor is especially friendly on Mac, though I think it might be available cross-platform with CSS as well (cursor:grab and cursor:grabbing), and it wouldn’t hurt to give Windows users the same added clarity—Steve Jobs once aptly compared providing Mac experiences on Windows to giving those users “a glass of water in hell.” :)
The three dots should never be used for drag and drop since it is typically used for secondary functions in mobile menus. An ellipsis (…) is the origin of the symbol, which indicates that there are more functions to choose from, as in Android’s action bar. The symbol has also been adopted on iOS to indicate secondary functions in apps as well, most notably on posts in the Facebook app. Symbols used in UX should carry well-known meanings rather than serve as decoration.
Depending on the direction of the drag, a double line should be used: || … horizontally, this was the original symbol used on iOS and evokes the shape of a handle that can be dragged. If the two lines aren’t clear enough, a skeuomorphic visual cue could be considered instead. At least on desktop, the cursor changes when you hover over it to minimize confusion. On mobile, you typically don’t see drag and drop unless an edit mode is enabled first, allowing users to derive the meaning of whatever symbol you use for drag and drop (hopefully the standard ||).
Looking at Mozilla’s developer notes, I see the || symbol is part of the col-resize and row-resize cursors as well, so it may predate iOS.
