How should good worksheet questions be written?

Short answer:

Clear and concise, with the goal of forcing students to think and not just repeat an answer they've encountered elsewhere.

Long answer:

Follow these general principles for the Question itself:

• Write the question as clearly as possible. Avoid confusing or ambiguous language. This does not mean "dumbing down" the problem itself, but rather taking extra effort to make the wording clear and unmistakable.
• When possible, choose questions that have more than one correct answer. Such questions teach students to open their minds to alternate possibilities.
• When writing a sequence of questions, try not to betray solutions to previous questions in the wording of subsequent questions. This is one facet in which written Socratic questions differ substantially from spoken Socratic questions (where the student has no prior knowledge of what will be asked next).
• Strategies for really thought-provoking questions include asking students to derive mathematical equations from given data, devising experiments to test hypotheses, and asking lots of questions beginning with the word "Why" or the word "Explain." Deductive thought -- proceeding from a general rule to a specific instance -- is easy. Inductive thought -- deriving a general rule from specific instances -- requires more thinking.
• The more practical the question, the better. For those instructors with lots of real-life experience in their field, feel free to document some of the more challenging problems you've encountered in the form of worksheet questions. This will enrich the educational experiences of students everywhere, and preserve your hard-won professional wisdom for future generations.
• Write each Question so that it makes sense on its own, and does not have to follow or precede any particular Questions in the same worksheet. This ensures fully modularity of the Questions, so they may be freely selected in the creation of new worksheets. If you absolutely have to have one Question follow another, embed it as a "follow-up" question within the Answer of the Question you want it to follow. In other words, write two (or more) Questions within the same question file.

And regarding the Answer . . .

• Answers should be "short and sweet." Resist the urge to provide lots of detail. Give just enough information to let the student know whether or not they understand the concept, without openly betraying the means of solution. Remember, you're writing a tool for inquiry, not a textbook to explain things!
• Feel free to include additional Questions in the Answer. This is in the tradition of Socratic discussion, where answers are followed by more questions. The Answers to these follow-up Questions, of course, will be revealed in discussion, rather than on the worksheet.
• Feel free to answer a Question with another question, so long as the other question legitimately addresses the first, as opposed to creating a new "tangent" of thought.
• You are not obligated to write an Answer for each Question! For some questions, it is better that the students be given no hints at all, but wait until the discussion time to compare findings with their classmates. In cases like this, I usually write something like, "I'll let you discover the answer to this question on your own."

Writing useful Notes:

• If writing for inclusion on the public website, do not write anything in the Notes section that you would not want a student to see (i.e. detailed solutions to problems, sources of information, etc.)! If writing your own private Notes, feel free to elaborate, as you are under no legal obligation to make your writings open to the public.
• The Notes section is a great place to add comments about teaching Socratically, and about common conceptual difficulties students have while learning the material.