I agree wholeheartedly with Alexandre's answer, but there is one other principle I'd add to his list which I believe is essential.
- Whether you merely cite the problem, include some small hints, provide copious hints, or give a full solution, should roughly correspond to the difficulty of the problem.
Of course, you can only know how difficult an exercise is if you have done it for yourself. Some exercises really are easy to experts in the field. Others are extremely difficult. Some are impossible.
And sometimes problems are just wrong. Indeed, one of my papers is a counter-example to the first two exercises in a well-known text.