At the risk of stating of the obvious, a mobile phone is not a laptop computer.
So when you are designing a mobile app, keep in mind that your users may not be sitting at a desk, but rather, they might be wedged between two people on a train, balancing themselves with one hand and using their phones with the other.
Coming up with an idea for you mobile app is just the first step; creating a user experience that brings your idea to life in the hands of your users is a much more difficult challenge.