Writing good essay assignments has to be about the hardest part of teaching first year writing. (Grading is more miserable, but not usually "hard" per se.) Actually, it's hard whenever and wherever you do it.
I "look for" a couple things in an essay assignment:
1) It has to be do-able, in the time allotted, with the resources we have available, and with the skills I can teach or realistically assume they already have.
This seems simple, but it's really not, especially for first year students. Some of my worst early failures at writing essay assignments came because I somehow thought students should actually know enough about their other classes to research information from them. But really, most introductory classes don't teach students to research in a given field, and I'm not really up on teaching researching in, say, physics, or intro to psych (because you KNOW they're not reading Lacan!).
My test here is that I should be able to write the assigned essay with ease.
2) The essays should NOT bore me to death. In practice, this means I have to change up assignments sometimes just out of a sense of self-preservation. But it also means I just don't open up areas for certain kinds of topics.
3) The essays should be miserable to plagiarize. I can't prevent all plagiarism. I especially can't prevent a student from paying a more advanced student (or some unethical business) to write a paper for a specific assignment. But I can do things along the way that will add work to the plagiarist's life (requiring students to turn in rough drafts is one step).
I can also do things that make assignments quite specific to my class. (For example, when I have my first year students write a movie review, they have to refer to a couple of professional movie reviews in a fairly specific way, and also include copies of those reviews in the turn-in packet.)
And I can hope that my students can't really afford to pay another student to write a paper for them.
I generally know what I want out of a given essay assignment, but communicating that to students is difficult, especially early in the semester or academic year. Usually I try to build skills as the semester progresses, so that early on, I want to work on analysis of their own experiences. Later, I'll ask them to read other evidence, especially other people's writing and work with it, responding to it, analyzing it, and finally using it in an argument.
But it's torturously hard sometimes to get them to analyze their experiences rather than to narrate or explain their experiences.
How do you teach students to analyze their experiences?
How do you discourage plagiarism?
One of the BEST things I've learned from a colleague (because, really, learning from colleagues makes life so much easier) about getting better essays is: just before the rough draft is due for peer revision, I ask students to reread the essay assignment, and imagine what a competent (as in a C) "response" to that assignment will look like. They work first in groups, and then we put ideas on the board.
Early in the semester, they tend to want to focus on "good grammar" and "no spelling errors," both of which are nice, but really aren't key to a competent essay.
I prompt and push until we get a list on the board which includes: a thesis, an argument, points that develop the thesis, evidence, example(s), an introduction, a conclusion, and, most important, whatever specifics the assignment asks for. If the assignment asks them to compare/contrast, then the essay actually has to compare and contrast. (I don't give compare/contrast essays in first year writing, but you get the point.)
Then when they start to work on peer review, they can at least use that "competency" standard to provide feedback about how well the essay meets the requirements of the assignment.
Bonus! The blogspot spell check wanted me to replace "Lacan" with "lacunae." How fitting is THAT!