And knowing your audience is critical. Fulfillment is a huge issue nowadays. Amazon's fast service has created very high expectations among online shoppers. You need to tell users how long it will take for a CD to be delivered, or they'll get frustrated when it doesn't turn up for three weeks. When it comes to asking a user to enter information, such as through a form, you must think about the kind of technological solutions you'll use.
Web users are incredibly fickle.
Privacy is a major issue, especially for more experienced users. If forced to enter information they suspect will be used for advertising purposes, they may lie. When it comes to the actual people who are coming to your site, you may choose to try and capture some personal information. Indeed, if you're trying to recruit users for research or usability testing, you'll have to find a way of deciding whom to recruit. This is often the information that helps you find the right people.