If you're ever shown a UI design for a new top-level application window, be sure to notice whether the controls on the window happen to perfectly fill up on the available space. This is often a sign of trouble.
Many application windows are resizable. (If a top-level modeless window isn't resizable, it's a reasonable question to ask: why not?) A common mistake when designing resizable windows is to focus too much on some perfect window size that happens to show off the window's controls to best advantage -- a window size that few people other than the designer is ever likely to see. It's important to question how the window is going to respond when the user resizes it. For example, many users commonly maximize the application window they're working in. A surprising number of applications actually look awful when they're maximized on a typical large display: the additional space the designer didn't design for is either wasted or allocated to some control (often a text box or list box) that didn't really need it.