I love video games, especially fantasy mmo's. Naturally, this leads to me including stuff from mmo's I like into my games. The easiest stuff to steal is place names. Just wandering around WoW will net you a ton of towns and mountain ranges and rivers. I also like to convert the occasional monster into my games. They can be fun, and my friends love seeing something like a murloc pop up, as long as I don't overdo it.
While I have no interest in playing a game set completely in the World of Warcraft, I see no reason why I shouldn't use the cool stuff. Where I will have issues with it, is when it affects the actual rules of a game. Having power slots that can be upgraded, tank/dps/healer roles spelled out, etc., irritates me. Yes, I can see how it would actually be helpful if I was just learning D&D or something similar on my own. But I'm old and cranky, so I don't need that in my games. It's my own little bit of curmudgeonism. :)