on a more interesting note, here's an email i just wrote:
I'm going to layout what I think bad is. I'm aware my answer doesn't tell us everything we'd like to know.
To start, we need to examine what a stable worldview is. A worldview can be said to be stable if new conjectures, new observations, new criticisms and arguments, won't send it off in new directions or otherwise cause it to change. A perfectly stable worldview would have to be entirely consistent, and entirely complete, otherwise it could be changed by new ideas.
Next, we ask what sort of stable worldviews can exist. I propose that there are three. The true, inverse, and null ones (alternatively: good, bad, and empty). The true one is stable because it's right about everything, and understands everything. The inverse one is stable because it's exactly the opposite of the true one, and persistently misinterprets all new ideas in the opposite of the true way, so that they are consistent with the inverse of the truth. The null worldview is stable because not only does it not say anything, but it can't learn anything either. It never hears of a new idea.
None of these perfectly stable worldviews exist (unless you feel like saying rocks qualify for the null view). They aren't real. But they can be approached. In practice, good ideas approach the true view, evil approaches the inverse view, and nihilism and relativism approach the null view.
Anyway, what is bad? Well, the ultimate in bad is the complete inverse worldview. And also, there can be lesser versions (ones with inconsistencies, and ones that don't yet deal with all subjects).