RPG settings are truly fantastical, with incredible terrain and inhuman opponents. As a result, the language we use in our everyday world requires careful consideration in RPG writing. Be aware of the following points, which I see from time to time and occasionally make myself:

Killing the Dead. You can't kill dead things, or even undead things. Undead don't fight "until slain" or "until killed"; they fight "until destroyed" or similar. The same goes for constructs, robots, or similar. Whenever you use words like kill, dead, or slay, think "does this apply to this creature?"

Stirred, Not Shaken. Many game mechanics are tied to ordinary words. Beware of these. While "armor class" isn't a phrase you'd use in casual writing, you should be careful saying someone is "shaken by what they've seen" or that a monster is "fiendish." Many ordinary words like "shaken" and "fiendish" imply game mechanics that you might not mean. Grab a thesaurus and look for another word instead--and be sure that one isn't a game term, either!

Monsters are Monsters. We use a lot of metaphors in daily speech. My wife is an angel, my neighbor's dog is a monster, and that guy at the gym is an animal. Many words we use as metaphors have specific game meaning (I can't cast charm animal on the guy at the gym), so avoid them in RPG writing unless you mean exactly that creature. 
Unholy Holiness. Many of our words to describe the supernatural have positive connotations. Something benevolent is "holy," "sacred," "divine," or similar. In a game with angels, demons, and gods of all alignments that answer prayers, be a little more careful about words you use to denote the supernatural. Evil supernatural things shouldn't use these positive words, and neither should neutral supernatural things. Demons don't carry holy weapons. "Unholy" is a standard fallback word to use, but that's also mired in one part of the alignment spectrum. A more generic word like "a religious symbol" often works well.

Do or Do Not; There Is No Try. When talking about a character's action, particularly a potential action, be more precise in your wording than you otherwise might in normal speech. Characters shouldn't ever "make a check to steer the ship" or "try to hit the ogre." Don't say "the mayor may talk to the characters about the killings." Talk specifically about whether a character must attempt a check, succeed at a check, make an attack roll, or hit a foe. The mayor talks to the characters. Words like "make" are unclear and words like "try" or "may" add ambiguity that is a burden on the reader.