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.

But:

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.