I am currently reading Benjamin C. Pierce’s Types and Programming Languages (2002), and in chapter
18 – on imperative objects and classes – he offers an off-hand comment about why the class features in most OOP languages are so complicated.

As someone who have spent countless hours writing C++ (and to some extent, Java), this immediately jumped out at me:

The class mechanisms in real-world object-oriented languages tend to be complex and loaded with features—self, super, visibility annotations, static fields and methods, inner classes, friend classes, annotations such as final and Serializable, etc., etc.

The main reason for all this complexity is that, in most of these languages, classes are the only large-scale structuring mechanism. Indeed, there is just one widely used language—OCaml—that provides both classes and a sophisticated module system. So classes in most languages tend to become dumping ground for all language features that have anything to do with large-scale program structure.