Good writing should be clear, concise, informative, often persuasive - it should show some insight into the issue under discussion. Some people have more extensive vocabularies, and are more confidant with newly encountered concepts, but that doesn't mean writing without these things cannot be good. A friend of mine is now doing a post-Doc at an Ivy league University in the US, with a internationally respected academic. She is gifted in her field, and her PhD made a substantial contribution. Despite this, her writing is not good - but it is improving. If you want to improve your writing, my advice is practice. Go through what you have written, and ask yourself what you don't like, and why - and then try and improve it. Ask a friend whose writing you like to comment honestly. Ask an academic you respect for some feedback. There are plenty of ways to work on an issue like this one.
Also, always remember that good writing should cast a light onto the issue under discussion. Unless you are in politics, it should not attempt to obscure an issue in shaddow. Some Uni students write in ways that are very jargon-laden, full of complex polysyllabic terms, and produce a product that seems impenetrable, or obscure or prententious. This is not good writing - and if done to impress an academic, it will likely fail (to impress, I mean).
