Nothing, in and of itself. Just as there is nothing wrong with eating an ice cream, in and of itself.

But if you get into the habit of eating ice cream all the time, if you start eating more and more ice cream to the point where you are neglecting normal food, you will end up with a nutrition imbalance which will result in decreased mental ability, decreased ability to concentrate for long periods of time, and oh yeah you will get fat, which most people consider to be a bad thing.