What is a code challenge?
A code challenge is a competition for creative ways to solve a programming puzzle for an objective criterion other than code size. Challenges to produce the shortest working code should be tagged code-golf.
What makes a good code challenge?
Many of the guidelines for code golfs are equally relevant to code challenges. The most common problem specific to a code challenge is that its winning criterion is highly subjective (i.e. each answer is equally valid) or that multiple, conflicting winning criteria exist. If you are unsure, first propose your challenge on Meta to get helpful feedback for improvement before you post it here.
Good examples of keeping subjectivity to a minimum: