Most of the other slightly non-standard CAD/eCAD tools I work with (e.g. Diptrace, PTC Creo Elements) use the right mouse button to drag.
So when changing between these applications, it's slightly confusing/annoying that I need the middle mouse button to drag in QCad.
Even worse, the middle mouse button on most mice is the scroll wheel which is not really comfortable to hold while dragging.

So I'd propose an option to switch the function of the middle and right mouse button to allow users to choose the preferable option.