A thread at Testy Copy Editors offers a great discussion about which headline puns work and why -- and you can quickly see why the qualities of a good headline are so subjective.
It also includes this instructive, if dense, passage from "Headlines and Deadlines":
Two tests can be propounded for puns, whether in a headline or elsewhere. The first is whether each of the two meanings of the word forming the pun is appropriate. ...Add to that treatises on when to use inside jokes and what makes humor, and you've got a recipe for an argument.
The second test is based on the theory that the basis of humor is incongruity and unexpectedness. This means that the pun should not be obvious; it should not be just lying around waiting to be picked up ... The best advice that can be given to the headline writer is to avoid the pun unless he is convinced that it is exceptionally good. If there is one thing that most newspapers need, it is more sophistication. The bad pun, like the childish rhyme, is the mortal enemy of this quality.