I know many Christians have no problem with Harry Potter. However, the truth is not decided by the majority.
Harry Potter normalises witchcraft and wizardry. It makes kids want to do magic. It portrays the magical people (witches and wizards) as much greater and more important than the normal people ("muggles"). It is a "soft" start for a kid to start getting interested in this sort of thing, and may then lead into them being curious about participating in seances etc.
I know that many people will disagree with me, Christians are very divided on this issue. However I do speak from some personal experience and observation in this area, and am quite convinced of what I believe. Harry Potter is very well written - the stories are very gripping and interesting for kids. But having a gripping storyline and being "just a story" does not necessarily make something ok. Children imitate what they read. They want to find out more. Witchcraft is not something to joke about - it is a real thing. And it is very interesting - it is a way of getting power, and who isn't interested in that? If a child wants to find out more about what Harry Potter does, it is very easy for them to find out about witchcraft and get drawn into it, just do one net search, and once you are involved in that sort of thing it can be very difficult to get out. There is power in witchcraft, and when people see that it can actually work they get very interested. However it is not wholesome power, and draws people away from Jesus and firmly into the clutches of Satan, who doesn't let go very readily.
One of the saddest things I see in the world today is Christian parents who don't consider the implications of what their children are feeding their minds with. They let them read all about witchcraft, play around with witchcraft-based games, and then naively believe that somehow none of it will rub off on their children at all and the only things their children will remember about the supernatural is what their Sunday-school teacher tells them.