What makes a question a good one

In my experience, there really is no consensus about what a "good" question is. If I asked someone, why aren't plants black?, chances are that person would say, "That's a pointless question. It just happens to be that way, there isn't a reason." However, this question won the Molecular Frontiers Inquiry Prize, which is judged by a jury, including Nobel prize scientists. There are questions that I ask my friends that they find really interesting, but when I ask someone on Y!A, usually that person says, "Don't ask us to figure out how to do YOUR homework" or "You clearly don't understand anything about that subject. That's a pointless question to ask."

Do people do that because they honestly think a question is a bad one, or are they just trying to conceal their lack of knowledge and avoid looking like a fool?

If you want to ask a meaningless question, and there are many here, put some words around it to make it more possible to answer.

Negative questions like the one you quote are specially meaningless, for obvious reasons. You might as well ask why are inches red, or why are people green.

To put it in perspective here is a sample of some questions I've seen recently here.

Can microwaves have children?
Why doesn't the earth fall down?
Can ghosts be digested?
Why is why why and a why?
Why doesn't the earth fall down?
Are there any submarines that can go deep enough to pop out the other side of the planet?
Help,me,out.'i-do-wan2,no,abt,…
Why is alaska snowy when its right next to hawaii?
Why does the moon changes its shape every day?
What happens if you fall of the earth?
If earth is round, why doesn't water fall when upside down in space?
Should giraffes become mirrors?
What makes the sun float? Maybe the whole solar system is actually sinking?
Can potatoes shoot lazer beems from their roots?