Action icons have a noticeable outline as well as consistent positioning, perspective, and shadow. When possible action icons should be compound of a single element. More than two elements will make the icon to look too crowded and not usable at all.
Mimetype icons use a color, tone, and design concept for each distinct logical set.
Filesystem icons are colorful yet subdued, realistic yet fun. Note that these are previews of work in progress.
Devices icons are very realistic yet somber which provides clarity while not confusing the user with too many details. It is important to represent devices and media storage in a way that user will immediatly recognize them.
Applications icons, due to their very nature, have the widest range of detail and color use.