For myself, it can be a number of things that sometimes feel contradictory. I enjoy challenges that aren't hyper-specific in their subject matter and yet, I am often drawn to posts that ARE hyper-specific in their formatting. The specifics of the challenge are important to me, but I need to be putting my own unique thought into that.

An example of a good and bad challenge for me would be as follows:

Good:

Write a piece of prose/flash fiction under 500 words based on the quote "It all adds up to much the same on the plane of proof."

or

Write a poem in Iambic Pentameter on the subject of darkness and light.

Bad:

Steven is a 16 year old from Newport. Sick of his boring life, he decides to pack his guitar and go on the road. Any format.

(I won't write this second one out, but I am not usually a fan of being given a paragraph, or even a few sentences as an opening to a story that I should continue. I don't mind doing this sort of thing in real time with people, but not just me and one person, in this format.)

Be evocative. Be specific in your format and open with your subject matter. :)