Sometimes it is difficult to properly express the problem I have with Christianity. It isn't that it is so difficult to put into words, but it is difficult to get anyone to listen long enough to understand exactly what the problem is.
Some of the nicest, most kind, most gracious, and admirable people I know are Christians. They are the kind of people that, while not making me desire to be a Christian, I can look up to and try to emulate they way that they approach life. This isn't meant as a compliment to them; it's just a fact. They are wonderful people, and Christianity is a part of who they are as a person.
Alternately, some of the cruelest, most self-centered, most uncaring people I know use Christianity as a cudgel to verbally bludgeon others mercilessly.
One of the primary problems that I see with Christianity is that, by claiming the title, anyone is almost immune to criticism for anything they say, or do to others -- especially if the deed is done in the name of Christianity. It becomes impossible to say anything to someone that is abusing others as long as they are hiding behind Christianity because everyone else that claims the title of Christian for themselves will rise up in defense and claim that they are being abused for their Christian belief.
After you have calmed their fear that they are somehow the victim of some sort of anti-christian abuse, only then can you begin to discuss what the original problem was to begin with. At this point one of two things usually happens: either the person waves their hand and says that the mean Christian wasn't actually a Christian, or they dismiss the mean Christian as someone that just has a different opinion of Christianity.
Regardless of the direction the conversation takes, the person usually dismisses whatever claim has been made against the mean Christian and considers the issue resolved. But the underlying issue of using Christianity as a cudgel is left untouched.
People can do good things with Christianity or without it. I have tried diligently to believe that Christianity is neutral at worst and possible even a good thing for some people. The problem I keep coming back to is that too many people use Christianity as both a weapon and a human shield. They use a weaponized version Christianity as a sword to brutally attack anyone that won't defer to their opinion and then use the nice Christians to provide them cover when someone sees them as the oppressors that they are.
Despite my desire to not see Christianity as a general negative in society, I am beginning to think that the atheists that see Christianity as a net loss to society just might be right. Perhaps Christianity does do more harm than it does good in the world.