I've recently been doing some of every academic's favorite activity - marking student essays (papers). Here's a few observations on essays and on marking them.
1. Marking Essays is Subjective This is a bit of a truism: it's fairly obvious that not everyone will agree on how to grade an essay down to the exact mark. Unlike with, say, a multiple-choice exam, marking an essay is not a mechanical process. But it's easy to forget this when the marks are there in black and white (or red). It's easy to assume that an essay that scores 68 really is better than one that scores 66, or A is better than A-. It might not be. The level of between-marker and within-marker consistency in scores will depend on the markers, courses and subjects. But the agreement will never be perfect. I would say as a ballpark estimate that mark differences of less than 5 (out of 100) shouldn't be taken too seriously. 2. Style Is Very Important The style of writing - which includes everything from English proficiency to the clarity of expression and logical flow - is extremely important. In fact, in most cases it's more important than the content of the essay. A factually shaky but beautifully-written essay will be marked higher than an unreadable work of genius. This is because while essays are marked on both style and substance, a poor style makes the substance seem poor. If your writing is unclear, you'll be marked down twice for it: once for poor writing, but then again for having a 'disorganized and confused argument', because your argument will seem disorganized and confused, even if it was clear in your mind. Sure, this isn't fair: in an ideal world, markers would be able to look beyond style, and recognize the underlying substance, but the truth is we usually can't. Or at least, we don't have time to do that. If it takes time and effort to work out what you're trying to say, most markers will assume you're not saying anything. 3. Essays Are Artificial Don't expect that writing essays is preparing you to write scientific papers. I don't know whether papers in the humanities are essay-like, but in science they are far from it. For one thing, you will rarely if ever be writing papers alone as a scientist: instead you'll be coordinating and negotiating between multiple authors, something that essay-writing doesn't prepare you for. Style is also less important in papers than in essays. There is an art to writing papers, but writing a good paper and writing a good essay are two different skills. Many papers in good journals are full of clumsy writing, but no-one cares, because the content - in the form of tables and figures - is strong. In less prestigious (but still legitimate, peer-reviewed) journals, it's common to see English and grammar mistakes that would get a student marked down badly. So, if you don't excel at writing essays, don't worry too much about your future prospects in science.