What goes against the commandments of God. Even if something is harmless, if it will lead you to commit sin, it should be avoided.
The 10 commandments involve two things: (1) service to God, and (2) treatment of others.
And things like getting drunk, sex outside of marriage, etc, are wrong because these things are harmful to us and possibly others; either health-wise, relationship-wise, or both.
I firmly believe that God's commandments exist for a purpose, and He didn't just say them to hear Himself talk.
Serving anyone other than God is wrong because we are to be an example to others, and to lead them to God. And if we are serving the fleshly desires instead of our spiritual ones, we will be much more likely to make wrong choices.
Some things are debatable as to whether they are sinful or not. I know my parents used to never let me watch any show that had magic in it--of course that changed after they approved of The Chronicles of Narnia and realized that it would be a double standard to approve of that and yet disapprove of other shows just for having magic in them.
Heck, IMO, whether curse words are wrong or not is debatable. I mean, how is one word bad when another word that means exactly the same thing is okay?
I avoid using them anyway, though, just in case. (Well, I try to. If I'm angry, I can say some pretty nasty things under my breath. *coughcoughcough* And that's definitely wrong; if not for the words, then for the wrong attitude towards and slander of others.)