1. Your Drug UseProbably every college in the country has to deal with substance abuse on campus, and most people who work at colleges have seen students' academic careers and lives ruined by drugs. If you've had problems with drugs in the past, even if you overcame those problems, the essay isn't the best place to draw attention to your use of illegal substances.
2. Your Sex LifeYes, sex is usually a bad essay topic. The admissions officers probably don't care whether or not you have an active or interesting sex life. More importantly, an essay on your sexual experiences is going to make many readers cry, "too much information!" You don't want to write about something that might be embarrassing for your reader.
3. Your Time in JailLots of successful students have had run-ins with the law, but it's not something you want to draw undue attention to in your application. The admissions staff is always working to create a safe campus community, and the image of you sitting behind bars isn't go to work in your favor.
4. Your HeroismSure, if you acted heroically in some way, it's a fair topic for a college admissions essay. It becomes a bad essay topic when the essay is self-absorbed and arrogant. I've read a lot of annoying essays about how an applicant single-handedly won the football game or turned a friend's life around. Humility is more pleasant to read than hubris.
5. One-Track Social, Religious or Political LecturesBe careful with divisive issues like abortion, capital punishment, stem cell research, gun control, and the "war on terror." You can certainly write an excellent and thoughtful essay on any of these topics, but too often than not applicants stubbornly and closed-mindedly argue what they see as the "right" side of the argument. The readers of your application don't want to be lectured to, nor do they want to be told they are wrong. The chances of offending your reader are high with some of these touchy topics.
6. Woe Is MeWriting can be excellent therapy for working through difficult and traumatic events in life -- assault, rape, abuse, incest, attempted suicide, cutting, depression and so on. However, you don't want your college admissions essay to be a self-analysis of your pain and suffering. Such topics might make your reader uncomfortable (a fine thing to do in other contexts, but not here), or they might make your reader question how ready you are for the social and academic rigors of college.
7. The Travel JournalColleges like students who have traveled, and travel can lead to a life-changing experience that could make a great college essay. However, travel is a remarkably common topic for college essays, and it often isn't handled well. You need to do more than highlight the fact that you have traveled. A travel essay should be an analysis of a single and meaningful experience, not a summary of your trip to France or South America.
8. A Comedy RoutineThe best essays often reveal a writer's sense of humor, but the jokes shouldn't be the point of the essay. Don't use the essay to showcase how witty and clever you are. A good college admissions essay reveals your passions, intelligence and strengths. A 500-word comedy routine doesn't do this.
9. ExcusesIf you had a bad semester or two in high school, it may be tempting to use the essay to explain your low grades. Perhaps you were ill, your parents were getting divorced, your best friend died, or you moved to a new country. You will want to convey this information to the college, but not in your essay. Instead, have a guidance counselor write about your bad semester, or include a short supplement with your application.
10. Your List of AccomplishmentsA college application gives you a space in which to list your jobs, community involvement and extracurricular activities. Don't use your essay for repeating this information. Redundancy isn't going to impress anyone, and a tedious list of activities isn't going to make a good essay.