Wanted to add a little more advice:
***Don't take huge risks***
I'm sure you've all heard that urban legend about the guy applying to Harvard/Yale/some other top school, responding to the essay prompt "What is courage?" with the single-sentence essay: "This is," and getting accepted. As far as I can tell, it's all myth: snopes.com: One Word Exam Answer
Of course, I'm sure people have tried it since those stories have gone into circulation.
Anyways, there *are* going to be cases where students take risks on their essays and get in, but such acceptances are rare. You have to consider that taking these kinds of risks only really work at less-selective schools that focus more on your academic stats in the first place. However, if you've already got the goods to warrant an acceptance, why even take the risk? At worst, it can signal to the admissions officers that you're being lazy and not taking the application process seriously. At best, you're taking a risk that gets you a result you didn't need the risk for in the first place. Just write the essay instead of trying to game the system with gimmicks. You don't need risks in order to stand out.
***Don't write a sob story***
Sob stories put the reader in an uncomfortable position. The reader has to wonder if the story is simply meant to garner sympathy, or if it's meant to genuinely reveal something about the applicant's voice/personality.
Like YoHoYoHo said, the point of the essay is to make the reader enjoy the experience so much that they want to meet you. Sob stories make this goal very hard to pull off.
Sob stories, typically, are seductive options to the writer because it's a chance to show a unique instance of overcoming strife and adversity (and believe me, I know the feeling -- I lost my father to a car crash back in the day). However, the essay is not the best place to demonstrate this. It's important to let the admissions officers know of huge setbacks, but it's something best left to your guidance counselor/teachers/etc to mention in their recommendation letters. Put your essays to better use.
***Word limits, do they matter?***
In a word, yes.
It's like that old quote, "I didn't have time to write a short letter, so I wrote a long one instead." Good writers know how to edit/cut/say more with less. Have you ever tried that experiment where you write a two-page paper and then try to cut it down to a one-page paper without losing any crucial content? The smaller essay is almost always superior. Furthermore, staying within the word limit shows that you can follow directions.
Will it hurt if you go over by a little bit? If it's just a couple words, then probably not. If it's entire paragraphs, then it's much more likely. Either way, why take such a needless risk? Play it safe. Stay within the limits.
***Is it OK to write about something controversial like religion/gun control/some other hot-button topic?***
I'd advise against it. While it's possible to write good essays about these things, you don't want to take the risk. Admissions officers are human -- not infallible pinnacles of objectivity. Besides, there are better things to write about for a college essay anyway, in my opinion.