qgis/QGIS ? reverse-engineered prompt

Build me a free, open source map and GIS app that works on Windows, Mac, and Linux.

I want users to be able to open common spatial files and connect to spatial databases and web map services, then view those layers on an interactive map, edit data, and reproject things on the fly. It should also have a strong map styling experience, with labels, legends, color ramps, printing and export to PDF, image, and SVG. Please include basic analysis tools, a way to run larger geoprocessing workflows, and support for temporal data and 3D view if that fits naturally.

Make it feel polished and usable for both beginners and power users, and leave room for plugins and scripting so it can be extended later. If you need to check current docs or best practices for any GIS related pieces, look them up online first.