Ultimately, it is the quality of the thesis that sets apart good essays from great ones. An essay that meets the requirements of a sound argument that has been articulated coherently in correct language should typically be a good essay or maybe even a very good one.
However, a good essay graduates to greatness only if it has a compelling thesis.
A compelling thesis is one that is provocative in intent and is worded in clear and crispy language. 'Provocative' means that the position it expresses on the topic is a healthy combination of fact and opinion: more precisely, a fact-enriched stance.
'Provocative' doesn't at all imply an eccentric or incendiary stance. At any rate, in the examination setting, you should not risk a controversial thesis statement.
'Provocative' as I use it here simply means that you should walk the rope between fact and opinion in such a way that your thesis sounds neither completely agreeable to everyone, nor entirely disagreeable. Indeed, it is the hallmark of a robust thesis that the 'disagreeable' element arouse the curiosity of the reader in a favourable way.
Further, a great thesis does not compromise on clarity. But clarity doesn't necessarily mean being familiar and self-evident. In fact, if a thesis is too factual, it may lose its 'provocation'. The clarity in question refers to the freedom from unintended interpretations. The core stance presented in the essay should for all its expressive sophistication have a singular propositional meaning.
In brief, a great thesis goes beyond simply being correct, clear and crisp. These qualities are indispensable but what makes the thesis great is the quality of the articulated opinion embodied in it.