Whenever an essay assignment asks you to argue for or against a given proposition, it is expected that you will give a clear statement - your thesis statement - of your position, somewhere in the first paragraph of your essay. But before we get into what makes a good thesis statement a bad one, there are number of myths that must be dispelled:
It is a myth that every paper needs to have a thesis statement. Some assignments will ask you to write your personal response, or explore a subject without judging it before hand. This is common with literature assignments where you have to be aware of multiple effects on the text instead of presenting a single position about it.
It is a myth that thesis statements must come at the end of the first paragraph. While this is the natural, logical and most common place to find such statements, it is certainly not the only one. In some cases, the thesis can be the opening sentence or at the end of the second paragraph if an introduction or explanation is required. In some cases, you can only formulate a thesis at the end of the essay.
It is a myth that thesis statements must be one sentence only. Clear, understandable writing is more important. If you must, use two or more sentences to state complex arguments. You can even write an entire paragraph (if it is necessary) to clearly state the position you wish to adopt.
With those common myths dealt with, we can move on to what makes a good thesis statement. These statements must give at least three point of support/evidence to support a given position. It is these three points that will be elaborated upon in some depth, but not limited to just these few points. The statements made must be definite and limited assertions that need to be explained and discussed in depth, as shown in the example below:
The final scene of Shakespeare's Midsummer Night's Dream employs subtlety in its linguistic and theatrical references to Elizabeth's position as queen to ensure its success.
This example clearly states the position, and provides the supporting elements that can be discussed in more detail, demonstrating a logical flow and progression of your argument. Compare this example to:
The world's greatest playwright was William Shakespeare.
This example merely states a position without providing any supporting evidence or arguments, and thus fails to be an effective thesis statement due to its simplicity and vagueness.
Thesis statements are not required for every type of essay assignment, but when required, they must not only state a position, but also provide the key points of supporting evidence that demonstrates a coherent argument. These are the thesis statements of outstanding essays.