Purpose
Like all things, websites exist to serve a purpose. Whether that purpose is to provide a simple public face while allowing you to manage your business behind the scenes, or to sell a product, or even if it's a community site to bring people together, it's important for that purpose to be well defined at the start of the project.
Yin
In most cases, the purpose of the website should be readily apparent to visitors. There's no need to drown them in it, but they should be able to tell fairly quickly what the website is about, and why it's there.
Yang
It's also possible to do too much to make the purpose of the site known, which quickly becomes tiresome to your users. You can't just push the end goal, you also need some other content there to temper it.
Balance
The key is to have enough content that supports what you're trying to accomplish to keep your visitors focused on it, but enough other content that they don't feel like they're being pressured in to action. This ratio is different for every website, but we know how to find it.
