Not to be confused with falling reptiles, the ability of a user to click on an object, drag it to some target, and drop it to make the target do something with it is both familiar and empowering, just as are other direct manipulation methods. Nonetheless, the drag-and-drop paradigm should be used only where it makes sense to do so -- rearranging items in a list, file management, opening a file dragged to your app from Tracker, etc.
If you plan on supporting dragging within your program or to other programs -- even just a little bit -- you should support drops to the most probable targets in your program. This shouldn't normally be all that difficult because drops to your program will come from either Tracker (in the form of an entry_ref) or from within your own program. Drops from other programs are not required, but if they are especially convenient, they can be a nice bullet point in a marketing blurb for your program.
When implementing dragging, be sure that your program provides feedback about what is being dragged. This is often best done by using an image which closely represents what is being dragged. If this is not possible, a rectangle which is the size of the dragged item's selection should be used in place of the picture.
A feature which is very helpful to a user is drop feedback. Any time the user is dragging something and the cursor passes over your program's window, it can react in a way to show the user that your program will accept the dragged object. Drop feedback can take a variety of forms:
There are other possible ways, but these should be enough for you to get the idea. As mentioned before, feedback is a Good Thing (TM). It allows the user to better understand how to use your program to get something done.