I think quantity is just as important as the quality, as most students are tasked to write an essay with minimum word requirement.
I don't have any experience as an online essay writer but I was once an essay editor, and I think there isn't much difference as being the actual writer. What we edit on essays is what should have been done/written in the essay in the first place. We take note of what's lacking, and what could be done to improve the essay.
That being said, the most important thing to consider, apart from the structure, is the content. This means that the essay must be relevant to the topic at hand. All information to be conveyed must be accurate so as not to confuse the readers. Moreover, the details must be authentic, as much as possible. For instance, technical knowledge must include technical details gathered from peer reviewed sources and must be properly phrased to avoid plagiarism. The essay must also be written depending on the tone that is expected of it. If it's informative, then it should contain as much information as possible. If it's persuasive, then it should be able to persuade the readers.
In every essay, the key is to write it as concise and as coherent as possible. It must be able to convey the message while meeting the word requirement.