I shop a lot inline because I live in a remote area and don't get to the LQSs as much as I'd like. I want a page that is organized and easy to navigate. One thing I really like is shops that organize their fabrics by colors, manufacturer, designer, etc. That way, if I've seen a picture of a fabric, but don't know all the details, I can usually find it quicker. Plus, I have certain designers I love (like Holly Taylor), so when I see their name, I can go straight there.
After I've ordered, customer service is most important. I want to receive my order in a timely manner- or hear from the website owner as to what the problem is. I also appreciate online stores who give you the option of "send me what you have", "call me if you can't fill it", etc.