Argumentative. It makes a case. That's the biggest difference between a thesis and a topic — a topic is something like “Slavery in Huck Finn.” That's not a case, only a general area. A thesis, on the other hand, makes a specific case, it tries to prove something. One way to tell a thesis from a topic: if it doesn't have an active verb, it's almost certainly still a topic.
Controversial. That doesn't mean something like “Abortionists should be shot” or “George W. Bush's election was illegitimate” — it means that it has to be possible for an intelligent person to disagree with your thesis. If everyone agrees on first sight, your thesis is too obvious, and not worth writing about. It also has to be something you can reasonably argue about: it's not enough merely to give an unsupported opinion.
Analytical, not evaluative. A college English paper isn't the place to praise or blame works of literature: theses like “Paradise Lost is an enduring expression of the human spirit” or “The Sound and the Fury isn't successful in its choice of narrative techniques” aren't appropriate. That's the business of book reviewers. No need to give thumbs-up or thumbs-down; evaluate the work on its own terms.
About the readings, not the real world. Never forget that books are books and, if you're in an English class, you're being asked to talk about them. Many books are unreliable guides to the real world outside the texts, and it's dangerous to talk about, say, Renaissance attitudes toward race based only on your reading of Othello. Talk about Othello.
Specific. It's not enough to deal in vague generalities. Some students want to write their paper on man and God, or on the black experience in the twentieth century. Both are far too nebulous to produce a good paper. Get your hands dirty with the text.
Well supported. That's the key to the rest of the paper after those first few