The six signals of quality in software
Does the software technically perform as expected, all of the time? This means no bugs, no downtime, no errors, and so on.
Does the software react to input instantly, or as near as practically possible? For those tasks that must take longer, are they as fast as possible?
Does the person using the software understand everything?
Can the person do what they need to do with the software?
Can the person do what they need to do with the software as easily as possible?
Is the software as aesthetically pleasing as possible?