Vocabulary: It's not fair (just, equitable, impartial) that we should be judged on it, yet it's inevitable (inescapable, unavoidable, predestined) that we are. There are several kinds of vocabulary fails: using the wrong word altogether, using a limited and unimaginative range of words or using slang words too new to be acceptable in business, academic or otherwise formal settings. (Our use of the word "fails," above, might be an example of this).
Another word-choice sin, one that we don't think about as often, is talking or writing over the head of your audience. If you're well-educated, it might make you feel brainy to use the word "adiaphoristic," but if no one understands what you mean, you're not communicating effectively. The first rule of speech or writing is to get the point across. If you haven't done that, what else matters? (FYI: "adiaphoristic" is essentially "agnostic," or not having a strong feeling about an issue).
A fun fact about vocabulary: The average speaker has a working vocabulary of about 2,000 words. This isn't just true of English, but across most of the world's languages. What about professional writers? Let's theorize that their stock of words is, say, about 50 percent larger. That gets us to 3,000 words. Here's the fun part: William Shakespeare, according an analysis of his written works, had an astounding 18,000-word vocabulary! So if you have difficulty understanding him, take heart: Even people in his own day were challenged.
We don't expect you to have a five-figure vocabulary ... but we've created a quiz that might stretch the one you do have. Set aside your dictionary, rely on your brain and see how well you do! Good luck!