Most OSX programs can be installed by dragging the app to the Applications folder (or wherever the user wants them).
However, some software must be installed by launching an .mpkg package, acting more like the 'double-click and press next-next-next...' installation method under Windows. For example: Google sketchup has an installer, while Google Picasa is drag-and-drop.
Why is this?
Would it technically be impossible for those programs that use the installer approach to provide the 'standard' drag-and-drop approach? I would assume that any files that need to be installed in specific locations could be done so upon first launch of a drag-and-dropped program?