I’ve been having my Macbook for about 7-8 months now, and this problem occurs on a regular basis: The drag and drop functionality in Finder stops working.
After googling for a bit, it seems Quicksilver is the problem. Drag and drop only goes away if QS is running, according to this forum thread. It seems the number of items Quicksilver is cataloguing might have something to do with it – it would certainly fit my profile, as I have about 70.000 items in the catalogue. Apparently, this can cause Quicksilver to experience errors and affect the Drag/drop functionality.
This post will be expanded as I look for solutions. Please leave a comment if you’ve also experienced this issue, or if you know of a workaround / solution.