There's probably a lot of such, taken for granted in most modern OS, however here's some that I like to mention here often, I just now compiled some of those that I remember into one place.
- ability to use common good UI-design tools on top of any custom engine (= low level overlaying/composition where layers can be running at various frame rates / freesync/ g-sync)
- low latency I/O for audio and user inputs across the OS - eg. suitable for realtime data acquisition and triggering external hardware on realtime schedule and minimal drift/jitter hardware osc/clock in the motherboard
- easy to implement jitter free smooth scrolling (one possible benchmark is, how much invidual pixels you can get updated smoothly at fixed low enough interval, eg. 20 ms, while doing useful work in other cores)
- security/tamper proof
- code transparency/readability after compilation instead of 'read the source'
- dynamic software updating - like JRebel ("Edit optimized code while running" - schedule updates to happen such that audio/gfx will run without audible/visible glitch)
- for virtual machines the ability to optionally patch old snapshots with new updates so that the patches don't need to be applied to each snapshot separately after each snapshot recall - essentially a snapshottable virtual desktop or snapshottable process
The last is no different from being able to save a game state, patch the game engine, then resume the saved game without things getting corrupt.. Yeah "mission impossible" if the game/engine/OS ships so buggy that it needs major patching. So this would need user visible patch classification so that user can decide whether they want to lose their state or not and be ideally be able to patch all security issues without losing state.