How is the OS suppose to know when a process is doing useful work? It seems like it needs to both guess the intentions of the application and the user using the application, and it has a high probability of getting it wrong.

Well, not until it has SkyNet levels of AI and mind reading capability built in. Maybe it can also post on Channel 9 for me too. Oh and take over the world and exterminate the inferior carbon-based lifeforms. But that OS would probably come from Google, not Microsoft.