Moodle's usability is great, but I'd like to suggest one small thing to improve it.
Clicking radio buttons for multiple choice questions is a bit slow because the radio buttons are so small.
If Moodle used <label> elements, users could click on the text of the descriptions or answers associated with the radio buttons, and the radio buttons would be selected.
Jakob Neilsen describes this on http://www.useit.com/alertbox/20040927.html in Point 11 of his usability guidelines.
The code is very simple; e.g.
<input type="radio" name="sample" id="one"> <label for="one">One</label>
Clicking on the word 'One' selects the radion button. I tried to put a working example in this message, but it got filtered out.
Hope this helps. I'm afraid my PHP isn't up to suggesting any code.