Before we start with the list of key parameters in building a great UI design for mobile applications, I would like to state it all depends on the target market of the app. Because we ought to create a product that is easy to understand and high on usability ladder.
Now, let's start with the list of key parameters.
1. Simplicity: Yup, a simple UI is of the type which permits the user to perform multiple actions with the help of a few steps. For example, using effective colors appropriately, large clear fonts, contrast for view, balanced shadowing, etc.)
2. Iteration: In order to retain the attention of targeted users for a longer time, it is a good idea to make use of iteration in UI design options.
3. Fully Responsive: UI design should be such that the app is fully usable on multiple devices and mobile operating systems.
4. Graphics fit to the Screen size: This implies that for your app to look phenomenal, include graphics that are tailored to the screens of the specific devices.
5. Right-Color Picking: It is important to use the right combination of colors in your UI design keeping in mind the app functionality and target audience.
Hope this helps. Suggest if something important remains left out.