Good math problems involving fractions are those that students can relate to their own experiences, making them more interesting. For example, splitting pizzas among friends or sorting a bag of M&Ms by color make good examples for teaching fractions.
Other than the obvious advice given to keep any educational problems at the appropriate level, problems that tie closely to the concepts the students are studying are preferable. Word problems with fractions about things not easily divided are obviously less preferable. For example, five friends splitting two pizzas makes far more sense than five siblings splitting two ponies. Even the wording chosen for particular objects in the stories can either engage or alienate students.