Before tackling a writing assignment of any kind, the writer must pay due diligence. This means that irregardless of the writer's knowledge of the subject, she must still employ research to use within the essay. Writing without researching is much harder and consequently puts the writer's credibility at stake.
The thesis is the focus of the entire essay, and it must clearly state the intentions of the essay as well as narrow the topic to something workable within the context of the essay. Without a clear direction or definition to base the thesis on, the writer is uncommitted and sacrifices the unity of the essay. Thesis statements broad in scope not only confuse the audience by offering too many generalizations of the topic but also confuse the writer. The essay then lacks focus and provides the audience with nothing to take away from the overall essay.
The structure of the essay stems directly from the research and thesis. The research provides the content, the thesis focuses that context and the structure gives the subject matter unity and comprehension. The structure involves discussing the essay in the way your thesis presents the content and in a logical manner that builds on the previous content. Often, essays with well-focused theses become incoherent gibberish because the structure was confusing.
Nothing kills a well-defined essay faster than bad grammar. No matter how well researched the essay is, how focused the thesis is or how balanced the structure is, essays rife with spelling mistakes and grammar faux pas quickly lose the audience's trust. Many of these problems stem from an overuse of passive verb forms ("to be") and structures ("there is"). Another major problem is using negative statements, such as "there is no peace." The positive form would be "there is war." Positive statements are easily comprehended by readers, but negative statements work sparingly to produce a certain effect intended by the writer.