Googlebot several links are missing.
- Links to user profiles
- Link to categories list from homepage
- Links to users activity from users page
- Links to related topics at the bottom of a topic.
- Any actual activity on users activity/topics page: Profile - sam - Discourse Meta (yes this looks the same as the users page because it is, no different content.)
EDIT: The idea being if a user would find it hard to navigate to / see related interesting info - so would a search engine.