Go 1.25 introduces significant improvements including the synctest package for testing time-dependent code with fake clocks, a major json/v2 package with custom marshalers and performance improvements, container-aware GOMAXPROCS that respects CPU limits in Docker/Kubernetes, an experimental Green Tea garbage collector optimized for multi-core systems, built-in CSRF protection, WaitGroup.Go method for simplified goroutine management, flight recording for execution tracing, expanded os.Root methods for safer filesystem operations, and various testing enhancements.