Some examples:
He ditched Windows Phone in favor of Android.
Microsoft had built amazing Office apps for iOS but Ballmer refused to allow their release. Nadella green lit them immediately.
He deemphasized Windows releases. Windows 10 was supposed to be the "last" version (no idea what happened after that tbh).
Everything Microsoft has released in the last few years has been cross-platform and cross-browser. This seems obvious now but would have been unthinkable a decade ago.
He doubled/tripled down on Azure.
Internally, he ended inter-division silos and forced teams to cooperate. Lots of old school execs were booted because they refused to give up control of their "kingdom". Engineering teams were also encouraged to use Linux and other non-MSFT tools and to open source their products. Culturally it was big to have an engineer at the helm again rather than a pure business type.