Grey WHAT!?

The most frustrating thing in today's software that's common to all applications, by design, and unnecessary, is greyed-out menu choices. I mean I understand the need for greying out choices, but the frustrating thing is that apps don't tell you /why/ a given choice is greyed out at a given time, and it's not at all clear in the majority of cases.

My idea is simple: provide tooltips when you hover over a greyed-out choice that give a brief mention of why the option is greyed out, and/or what you can do to rectify it. Applications could do this manually, but it would be easier if provided as a feature of the Windows (and/or even Linux) API, and that would also encourage the use of this new GUI feature.