Game design Toolbox – User Interface

Your UI Design Is Not Finished Until The Game Is Finished
The UI needs to be an elegant interface to the user, enabling him to do effectively what the game requires him to do. You cannot really do this until you have clearly defined what the user will do. So you can expect to revise the user interface design until it is clear what the user needs to do, and you have been able to confirm it. Early iteration is needed because you want to be able to get a feel for the game, but do not invest more than necessary. Since the user interface is by definition the closest thing to the user, it will probably require the most iteration.

Minimal Active Circle In User Interface When Using A Mouse
This only applies to the design of user interface that are used with a mouse.
It just happens that users are only that good at positioning the mouse and clicking, and the most annoying thing is to have very precise things to click on. For example you have a big button that brings a dialog, and a very small close box to remove the dialog.
I always ask the user interface designers what is the smallest circle that they want the user to click in their interface, and then ask them to make sure that there is nothing smaller than that circle anywhere in the user interface. That is really tough to achieve, but it makes for a more comfortable interface.