I think most people think they know what a healthy diet looks like, but in reality they do not. I see so many comments on MN that demonstrate this.
The most common one is that fruit should be limited because of its sugar content. This is very bad advice. Sugar in fruit has little impact on our blood sugar levels. And most people in the UK do not eat enough fruit.
The other is concern over eating any carbs. Wholemeal bread and pasta is fine, carbs in pastry and doughnuts is not great though.
And most people need to eat more nuts. Nuts are very good for you and should be part of your regular diet.