The Christian world is full of what Scripture refers to as "doubtful disputations" (see Romans 14). There are many things which, while permissible in the faith, are not actually requirements, and thus, one can happily enjoy them, yet also abstain from them. And indeed, there are good reasons to abstain. The aforementioned Romans 14 concludes with the words "...for whatsoever is not of faith is sin." Thus, a good rule of thumb in the faith is to never do anything you can't fully justify in your own heart.
The specific themes in Romans 14 are food, drink, and special days. In Christianity, you are 100% free to decide what you eat, what you drink, and what/when you celebrate. The sole conditions are no actual sin (e.g. you can't steal), and it must be in line with your personal faith. One could say you take the Henry Ford approach: "If you think you can, or you think you can't, you're right."
You also cannot compel other believers to agree with you. If you are a vegetarian, you cannot tell other believers "it's wrong to eat meat". If you celebrate Christmas, you cannot tell another Christian "you must celebrate too." In order to maintain unity within a diverse body, Christians must never judge or violate another's conscience.
Of course, the ultimate problem with this concept is how we distinguish between doubtful disputations and genuine moral issues. While Romans 14 gives us excellent examples, it doesn't give us a comprehensive list. The good news is, Scripture is quite sufficient to equip us for every good work (2 Timothy 3:16-17). There is nothing good that the Bible cannot equip us to do, and nothing evil the Bible cannot equip us to avoid.
We can take the 10 commandments as the obvious example. With the exception of the 4th commandment ("Remember the sabbath day, to keep it holy." - Exodus 20:8), which, as we have just seen, is explicitly included as a doubtful disputation in Romans 14, all of these are moral issues. You cannot blaspheme, murder, steal, lie, cheat, etc. Even if you personally feel these things are good, Scripture makes it very clear that these things are wrong, and so if you think they're ok, you are wrong.
But then we get to more complex, specifically modern issues. The Bible doesn't say too much about movies, or videogames, or rock music, or various other things, for reasons I consider obvious. Now, that doesn't mean there are no moral implications surrounding them. For example, we do have clear commands that would compel us to shun "adult" movies. Jesus tells us plainly "But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart." (Matthew 5:28). There is no way to watch (much less make) pornographic material without actual sin, and thus such movies are sinful. But what about movies with no such moral implications?
In my experience, Harry Potter is the most controversial series in the modern Christian world. The reasoning is that it (supposedly) generates a certain reverence for/celebration of the Occult. Now, this would be valid reasoning if Harry Potter genuinely was Occultic. However, the only people who say such things have a low understanding of either the Occult, the Harry Potter series, or both. Let's look at one comment I found on a Facebook discussion on the topic:
Can anyone who has seen the Harry Potter series dispute this? Of course Harry Potter isn't actual witchcraft. The witchcraft in Harry Potter is literally impossible in the real world. In fact, the very instruments used to perform magic cannot be replicated in real life. Wands contain parts of fictional animals. Phoenix feathers, unicorn tail hair, dragon heart strings, these are not things that can be found in the real world. Thus, the "witchcraft" in Harry Potter cannot possibly be related to the real world. And God is not in the habit of forbidding fictional things. If you can't do something, why would God even need to forbid you from doing so? Imagine it. "Thou shalt not sprout wings and visit Saturn for the summer holidays". This is what people sound like when they say Harry Potter is evil because it's about witchcraft.
And that really is the problem. When you oppose Harry Potter for witchcraft, you sound crazy. Not to mention hypocritical when you are later caught popping a copy of "The Chronicles of Narnia" into your DVD player. And much like the boy who cried wolf, you lose credibility when preaching on actual moral issues, and even when preaching on God Himself. When a believer hears you preaching against Harry Potter, and finds you have exaggerated the danger, they may not believe you when it comes to other issues. And when unbelievers hear us preaching against Harry Potter, they will look at us, see that we at least appear to believe in boggarts and grindylows, and wonder how they can take our belief in God seriously.
Now, obviously, this is not limited to just Harry Potter. There are other movies that are opposed, along with tattoos, piercings, rock music, martial arts, and a host of other things Christians really ought not quarrel over, both because of the damage it does to our credibility, and because the word of God literally says "And the servant of the Lord must not strive; but be gentle unto all men, apt to teach, patient," (2 Timothy 2:24).
If we are to be effective in our faith, we must set aside petty disputes over who has what interests. Scripture must be our ultimate authority. That which is explicitly forbidden, we must avoid, but barring some coherent case, that which is not explicitly forbidden is implicitly permitted. Any other attitude necessarily leads to petty squabbles, which damage both the faith of fellow believers, and our witness to the world. Let us therefore take these words into account: "Hast thou faith? have it to thyself before God. Happy is he that condemneth not himself in that thing which he alloweth." (Romans 14:22, emphasis mine).