Obviously that's much too broad a question. And when it is asked, people usually respond by pointing to the good and bad things people do in the name of religion—e.g., like helping those in need (good) and the crusades (bad).
But I think there is a real answer. A column by George Monbiot in The Guardian reminded me why, in general, I think religion is bad: at its core religion teaches people to favor faith over taking responsibility for one's beliefs and actions.
One can probably stop there. Is it ever a good idea to encourage people not to think for themselves? I doubt it. Even when people come to incorrect conclusions by thinking for themselves, one at least has a chance with them if they are open to the idea that one should think things through. Religion closes that door by closing people's mind. It encourages a perspective in which a given opinion is to be accepted no matter what—because it is God's will or God's word, for example. The point is not whether some particular position is or is not "God's will" or "God's word." The problem is with the idea that one should decide something by asking whether it is "God's will" or "God's word." That sort of thinking allows people to let themselves off the hook of taking responsibility for their own actions and decisions.
It's a lot easier simply to go along with the crowd or to do whatever one's religious leader says. That's true whether one is religious or not. But the problem with religion (and any cult) is that it encourages that sort of behavior. By its very definition, one of the fundamental teachings of a faith-based religion is mindless faith.
I'm finding it difficult to express how deeply angry I feel about this. A country whose citizens are trained to be meek (and sometimes not so meek) followers of their religious leaders will inevitably become a backwater of ignorance and stupidity. That's what religion is doing to this country, and I hate it for that.