The registry, NTFS, the security model, and the enterprise tooling behind Microsoft's OS.
Declaring a machine's desired configuration once, then letting PowerShell DSC continuously enforce it, instead of re-applying settings after drift.
Using Process Explorer and Autoruns to inspect a suspicious running process and audit every autostart location that Task Manager doesn't show.
Building a current App Control for Business policy from a reference system, deploying it in audit mode, reviewing evidence, and enforcing it in rings.
Deploying Windows Server Update Services to control which updates deploy where and when, instead of every machine pulling updates directly from Microsoft.
Microsoft retired the Internet Explorer 11 desktop application for Edge, while quietly preserving IE-dependent legacy compatibility through IE mode.
First previewed as Monad in 2003, renamed Windows PowerShell in April 2006, and released that November - replacing cmd.exe scripting with an object-oriented shell.
Announced at Build 2019 and pushed to GitHub May 3, 2019, Windows Terminal brought tabs and modern rendering to the command line, fully open source.
UAC was a genuine security improvement, but its constant, poorly-tuned confirmation prompts made it the most complained-about feature of Vista's launch.
Replacing the 17-year-old Start menu with a touch-first screen alienated Windows 8's mostly desktop-and-mouse users, and Microsoft reversed course fast.
Why Microsoft's April 8, 2014 cutoff for Windows XP updates became one of the most consequential OS end-of-life dates, given how many machines still ran it.