Confluence is one of the best designed products I've ever worked with. Makes the web behave like a native editor, awesome semantic macros that let you put in warnings or collapsible sections in a few keystrokes, you can generate page source with scripts.

Specifically regarding structure:

- every page has breadcrumbs letting you navigate up to its parents

- "child view" macro shows all children of this page automatically, so you can easily make a "category" page linking similar issues that is always up to date

- can move pages to new parents/spaces with a few clicks, or en masse with a drag-and-drop tree view

- built-in widget that searches only children of the page you embed it into

- spaces so you can tell a page in the search is owned by a different section of the team

Plus just a way lower barrier to entry than Wikipedia or the other wikis I've contributed to. I love Confluence.