It's also tricky because it's hugely important from a usability perspective. If your site's navigation isn't doing what it should, you risk losing visitors.
People flow smoothly from action to action, from place to place.
Like a dance, navigation has to be choreographed and tested, and this takes careful planning. In order to do this, you have to understand your audience's goals and needs. For example, let's say you run a bookstore. Your customers will come to buy books, but if you simply set up credit card processing and call it a day, you're missing opportunities. Talk to customers and find out how they want to find and purchase books.