Really, this question is a trap; almost like "tell us your weaknesses. You can always talk about the negative trends in dentistry. For example the growing amount of people with out dental insurance. Or the new trend for dentists performing more cosmetic dentistry than therapeutic treatment (I would only use this if you do feel strongly about the issue, and also check to see if the school feel's strongly one way or the other, I would not say this at NYU for example). By using these topics, you kind of side step the trap they are setting, but also appear knowledgeable. In addition, both of these topics are things that most dentists are going to agree with you on. No one is going to dispute that decreased amount of people with insurance is a bad thing.