How much time do you think the average visitor spends on the site per visit?

This question can help determine the width/depth of the site and the navigation structure, as well as the site features that are needed.
For example, if the site you’re building is a community-driven site, then you’ll want to consider some game mechanics that will help increase user participation and have them stay longer on the site.
Again, check this against real web analytics results whenever possible.