At present, not all the buttons on the UI have icons associated with them, e.g. "compile project", "Reset", "Run", "Test project", and "Javadoc".
When switching off of Metal look-and-feel, buttons in the same group do not have lines between them, making the buttons hard to distinguish from each other. For instance, it looks like there is a single button called "Run test Javadoc" rather than three separate buttons.
The Windows standard is to always have an icon on every button, which helps to separate one button from another (see e.g. IE).
This would make non-Metal look-and-feel be much more palatable on Windows machines, for example.