Just basic collision detection. Granted, it does depend a lot on the game made. If you make the game tile based, then it isn't necessary. Collision detection just involves testing if something is already in the tile. Anything that isn't using tiles, but requires collision detection would greatly benifit from a physics library (if for nothing else than the automatic spatial partitioning involved). Maybe something like OZCollide is better since it focuses purely on the collision detection. Just because a game is 2D doesn't mean it doesn't use physics or good collision detection. Just look at Angry Birds or Gun Bros for an obvious example of a (relatively) simple 2D game that uses a good amount of physics.

If the contest wants to stipulate that certain types of game be made, instead of just seeing what the developers create using the art, then that's one thing, but if the contest wants to see all manner of interesting games I feel a physics library is almost essential, and it would be a disservice to have an official library list that did not include at least 1 physics library.