A good essay should clearly define its subject.The reader shouldn't have to guess what you're talking about,because it should be obvious.Your essay should come out of the gate with it's mission in hand.It's purpose being easily understood by your reader.
A good essay should make an assertion.It must put forth an idea or opinion.Your reader should be able to agree or disagree with your assertion because it is not a fact,but a supposition,a view or judgment on a matter.
And last of all,a good essay should be written well.Even a good opinion or judgment can make a bad essay if written poorly. How can your readers follow your reasons if they become befuddled with bad grammar,spelling,or punctuation? Therefore,three traits make a good essay--a definite subject,a workable assertion,and a well written work.