At the surface, I'm bored. My mind is bored and I need to stretch it. So, I want prompts.
Underneath that is something more murky.
In my opinion, the goal of a good prompt is to target a weak point in the writing of most without stating the weak point you're targeting. If you're forthright, you risk giving away what you're seeking to get and coming out with nothing but a pandering audience. If you're too subtle the writers will miss the exercise and write what they feel - usually comfortable territory.
Give me a good prompt and I'll return the favor with one that's been brewing for a while.