According to UX Magazine:
UI elements are abstractions which convey concepts and ideas; they should retain only those details that are relevant to their purpose. UI elements are almost never representations of real things. Adding too much realism can cause confusion.
There is at least one specific area where more details are good: application icons. You want your icon to depict one specific idea: your application.
Coda’s leaf isn’t a representation of the idea of a leaf; it’s a very specific leaf, the Coda leaf. Acorn’s acorn isn’t just any acorn, it’s the Acorn. Adding details moves these images from a generic concept towards a specific entity, and in the case of an application icon, this is exactly what you want.