Simple writing is good writing. If you think people like you better because you use lots of big words, think again. The bigger your words, the less people understand. The less they even try to understand. The less they care. If they don't care, they won't give. So please keep your writing simple.
The above paragraph uses only words from among the 1,000 most common English words. I used The Up-Goer Five text editor, which immediately flags any word you use that's not in the top thousand.
It's an interesting diagnostic tool for simplifying your writing. Just don't think you need to use it to remove every word that's #1,001 or higher from your vocabulary. You'll end up with some really awkward circumlocutions. You'd be in deep trouble if you're a cancer charity: the word "cancer" is not in the top thousand. The editor itself has to refer to the top "ten hundred" words, because "thousand" is not among them.
However, if you check out your copy and find a majority of your words flagged -- you may have a problem: Copy that's hard to understand.
Thanks to Bad Language for the tip.