When writing, I tend to want to use words or phrases that have meaning in the real world, but wouldn't necessarily have meaning in the world in which I'm writing. For example, in a fantasy world, a character can't (metaphorically) "rocket across the room" because there is no such thing as a rocket.
One example that came up recently with someone reviewing my writing was "tug of war". "Do they have tug of war in your setting?" he asked. I was using it as an example of the effect made when one person let go of something another was trying to grab.
The main question here, I think, is this: Should I write as if the story is being translated from some unknown language into English (i.e. I can use words like rocket and tug of war without issue)? Or should I write in the language that the actual characters would be using in the story? The answer would be clear if the story were in first person, but since I'm writing in third person it's a little hazy.