One of the toughest things about writing is getting the language right. I don't mean style or the order of words - I mean the actual vocabulary.
You work hard to draw your reader into your world, to have them believe in it, and each time you use an inappropriate word, you fling him or her back into the present. The moment someone is jerked out of their belief in your setting and period, you - the writer - have to struggle to regain their trust.
Some things are obvious. You can't have a man living in a medieval world say "Ok". But lots of other choices are more subtle. Can you have him say, "Run that by me one more time?" (Not in my book, you can't. The phrase just sounds too modern.) Can you have someone in your made-up, pre-industrial fantasy world use the word "teenager"? Or does that sound too modern? Can the healer refer to a heart attack? Or a stroke? Or is he more likely to say apoplexy? Did Roman ladies wear "make-up" or is the word cosmetics better? I have just annoyed a reader by using the word "minutes" in a society that uses only sundials to tell the time. Appropriate or not? Not to that reader - it jerked her out of her sense of place, and that's enough to have me think I shan't do it again.
And then there's those foreign words which we use all the time - but are they appropriate? Can you say "deja vu" in your world? What about "run amok"? "An Oedipus complex"? Or "spartan"?
Sometimes it's the little things that count.