The point of OpenMark is this: to have an online computer-marked question, you really need to do two things:
- You need to display the question as HTML with form fields to capture the student's response; and
- then when you get the student's response, you need to grade it to generate a numerical mark, and some specific feedback to help them learn.
In an OpenMark question, you do 2. by writing custom Java code. This lets you create really sophisticated questions with really high quality specific feedback. The down-side is that you have to be able to write Java code.
In contrast, in Moodle, teachers control the grading just by filling in options in the question editing form. Therefore, the types of grading algorithm and feebdack that can be used are limited by what the question type allows.
This is not bad, because the question types have been designed to enable teachers to do the the things that people most commonly want to do, so it works fine 90% of the time (or 99%). However, for the rest of the time, you are stuck unless you have something like OpenMark.
Most of the new question types that the OU released a few months ago (http://moodle.org/plugins/browse.php?list=category&id=29) were coneceived by my colleague Phil Butcher looking at the quetions people had created in OpenMark over the years, and then working out how to let teachers create questions like that just be filling in a question editing form.