With the revised API, you can now use interactive Data Layers to collect user reviews on the best places to visit in an area, and display heatmaps showing the popularity of various locations.
Google also gives the example of building a tool for realtors, which allows users to draw polygons on a map to mark which areas they want to live in.
The input from these layers can be collected in GeoJSON format. Google has launched a GeoJSON editor to allow developers to import, edit and export this data.
Further documentation on these new features is available in the API reference.