Couple of UX/IA things:
- There's no hover effects on your links. This makes the top level links (Business, Discussion + Analysis, and Letter from the Founders) look like they're not clickable, especially because they're treated so differently than the rest of the links.
- Plus signs are often used to hide nested content, especially in navigation trees like you have here. If these are simply links, they should look like links.
- Once you're in a section, there's no breadcrumbs or active styling in the header. Because there are so many links, it's hard to know where you are or create a mental model of the space.
- It might be helpful to add previous and next navigation once you're inside one of the nested pages, instead of requiring people to go back out to the main page.