The system text editor is only capable of displaying text in system fonts. So, system fonts should always be used in input to avoid unexpected behaviour.
Default colors are taken from the system theme. Those colors may change between devices. For example, if you do not override all colors in your custom theme, it may lead to black text on a black background.
If you are planning to use a custom theme, make sure it is based on the Nokia theme unless you are going to override everything in your theme.
Low-end devices are not very good at full-screen transitions because the transitions require several drawing cycles.
If you have a fast animation, it might skip animation steps in slower devices and only draw the beginning and the end of the animation.
Smaller UI components are faster to animate than larger ones. For example, animating a textfield is faster than animating a long list.
Using 3D transition is not recommended. It is labelled as experimental and requires a lot of resources.