How well, not just what.
Features describe what the product does. Non-functional requirements describe how well it has to do it, and they're easy to forget.
Performance
How fast is fast enough, for which users?
Reliability
What uptime do customers need? What happens when it fails?
Security
Who can see and do what? How is it attacked?
Privacy & compliance
What personal data is stored, where, and for how long?
Accessibility
Can everyone use it, including with assistive tech?
Scalability
What happens at ten times today's customers?
Quality is a requirement.