The opposite of a focused, narrow, crisp thesis is a broad, sprawling, superficial thesis.
The sentence that captures your position on this main idea is what we call a thesis statement.
A thesis statement focuses your ideas into one or two sentences.
In short, instead of merely stating a general fact or resorting to a simplistic pro/con statement, you must decide what it is you have to say.
Tips: Do not expect to come up with a fully formulated thesis statement before you have finished writing the paper.