Good question! But I have no good answer.
1. no convoluted formulations.
2. no (severe? ) efficiency constraints.
3. correct, straightforward solution should be no longer than 100 lines of code (preferably around 10)
4. [edit: 2014-01-09] the matter of the problem should not involve advanced concepts.
ad 2) Yes, setting the problems please set: "Placement in main problemset" to "none" and please leave "Available for use in 3rd party contests" box ticked.