[immibis]

Implementation difference. Threads are usually handled by the kernel and each one has its own stack where you can put anything. And the kernel can switch threads at any time. Async/await has the compiler work out precisely what has to be saved across the marked locations which are the only places context switches can happen. Also it doesn't tell the kernel when it switches context.

[gpderetta]

Threads can be in scheduled in userspace and can also be non-preemptive or have deterministic scheduling.