This question is just out of interest, and perhaps could be useful for my thesis.
A web application, especially when your business is relying on it financially, needs to meet certain requirements in order to survive. I'd like to hear what kind of (software) quality attributes you find most important (name a few) regarding to web applications.
I give my share: performance (when visitors have to wait more than 2 seconds, 50% or more will leave your site, meaning less possible sales).
security , atleast be aware of OWASP
reliability, your web app must be able to handle faulty input, but also loads of them.
maintainability, in this world being able to change fast and adapt to your users need to increase sales is vital for survival.
This question is interesting and you have already mentioned interesting non-functional requirements (aka qualtities).
I think however that in the case of e-commerce website, the most critical qualities are the ones that impact the end user. My ranking would be the following:
The other qualities are important for the company powering the e-commerce website. But you could ultimately be successful even without them. My ranking would be the following:
EDIT: I just bumped on this other answer which showed the importance of low response time. So it's indeed a very important factor. Whether it's more important than security, usability and availability I don't know. The point is that we can measure performance easily, which is harder for usability and security. I would be interested to know how many customers are lost each time a popular website suffer a downtime though.