* It is closed to external scrutiny.
* Users are unable to add features to the software
* Users are unable to correct errors (bugs) in the software
The result of this is that proprietary software,
* does not confirm to good standards for information technology.
* is incompatible with other proprietary software.
* is buggy.
* cannot be fixed.
* costs far more than it is worth.
* can do anything behind your back without you knowing.
* is insecure.
* tries to be better than other proprietary software without meeting real technical needs.
* wastes a lot of time duplicating the effort of other proprietary software.
* often does not build on existing software because of licensing issues or ignorance
GNU software on the other hand is open for anyone to scrutinize it. Users can (and do) freely fix and enhance software for their own needs, then allow others the benefit of their extensions. Many developers of different expertise collaborate to find the best way of doing things. Open industry and academic standards are adhered to, to make software consistent and compatible. Collaborated effort between different developers means that code is shared and effort is not replicated. Users have close and direct contact with developers ensuring that bugs are fixed quickly and users needs are met. Because source code can be viewed by anyone, developers write code more carefully and are more inspired and more meticulous.
Another partial reason for this superiority is that GNU software is often written by people from academic institutions who are in the centre of IT research, and are most qualified to dictate software solutions. In other cases authors write software for their own use out of their own dissatisfaction for existing proprietry software - a powerful motivation.