Can't drag modules into an empty column
Once you move all the items out of a column, you can't drag anything back into it. The column has a zero height, so it doesn't have any hit area.
If you add a 20px bottom padding to the dashboard-column class in dashboard.css, the column has a minimum height of 20px, so you can drag stuff to it, even if it is empty.
The extra 20px also make it easier to drag a module into the bottom of a column. There isn't anything special about 20, maybe 10 or 50 would work better. 20 just seemed ok to me.