Is this a serious question? How about Customer Service? The old adage "the customer is always right" is still true. If you give me hard time with refunds and/or exchanges - I'll never come shopping at your (e)-store again.
Along with the above I'd say navigation and price. I need to be able to find what I want easily and without too much difficulty. And of course your prices must be competitive or I'm out of there.
There is no single most important point. Everything has to work together. A good website, proper SEO, good customer service, high-quality goods, etc. It is impossible to run a good online store with good quality goods without, for example, proper SEO.
Nothing is less or most important for an eCommerce website. Everything contributes to a website from product quality to aftermarket service. If the product is good then what is the benefit to have a great aftermarket service? You have to make everything in the right way.