I tried a little experiment this semester: I asked all three of my classes to develop an essay question for their respective final exams. (It will be one of several options, I hasten to add.) The students had a sample essay question in front of them and a list of requirements, ranging from the equivalent of "please make sure your question doesn't come out of left field (look at your notes!") to "please make sure you can answer this question in three bluebook pages/about thirty minutes." At the end, I typed up the questions, projected them on the screen, and had the full class discuss each option. And then, they voted. Ta-da, instant essay question.
In all three cases, one of the most frequent comments I got or overheard during the process was "Wow, this is hard." As an undergraduate, I certainly never gave any thought to the process of developing an examination, let alone coming up with five or six essay questions that were neither too big nor too small. One of the points I made during this exercise was that the essay should enable students to show off something that they've learned; I'm not simply trying to torture their fingers.