AJ gives a very detailed overview of Google Now Topics page and have cool observations worth looking into:
To me it shows that Google understands the intent behind each topic. So the topic of New York City brings up 'attractions and photos' while the topic of Googlebot just brings up 'articles'. Google clearly understands that Back to the Future is a movie and that I'd want reviews for the Toyota Prius Plug-in Hybrid.
Google Now Topics provide a glimpse into the Hummingbird Update by showing how Google takes words, queries and behavior and turns them into topics with defined intent.