Granted, this may be a little obvious, but ask yourself this one anyway. Stating what type of site you intend to have may give you a better perspective to work with when building it. There are a variety of sites and each one has a different business model. If your site is not commerce oriented, there are still ways to make money. Some types of revenue-generating models include ad banners, sponsorships, paid listings, and sponsored newsletters.
Functionality abounds on the Web.
Content management, database tracking, personalization, newsletters, real-time chatting, interactive personal guides, the list continues to grow. A very critical question and one you shouldn't rush into. Think of it as your brand. It must tell the tale of your site, be memorable, and easy to spell. If this is a brand new site that doesn't have an existing brand, then the domain you choose will be your brand.