The most important aspect of essay writing in a Psychology exam is to be able to critically evaluate. This means weighing up the strengths and weaknesses of the Psychological theory or concept you have just summarised.
As Psychology is a science and driven by new research, it is also really admirable to make judgements about the utility and application of the concepts being discussed. Putting forward your own ideas on how research can be improved is just as important as being able to name-drop researchers who have evaluated theories and models, so really make sure your own opinion is voiced in your essay.