Weak, dynamic typing. Dynamic method binding at runtime (method calls really are message-sends). 'Traditional' object orientation with inheritance (although delegation and dynamic mixins are also core concepts, and the class hierarchy and method lists can even be manipulated at runtime). Nil is allowed anywhere and messaging (calling methods on) it is a no-op. Heavy use of 'notification' posting and subscription.

All of these are unfashionable nowadays, but they’re fundamental to Obj-C (some to the structure of the language itself, some just as idioms) and to the design of AppKit and UIKit.

Now, the fashion is:

Strong typing. Static binding at compile time with no runtime modifications to the type hierarchy. If OO is used, there should be minimal inheritance. Nullability is strictly defined in the type system, and acting on null objects causes, at its most forgiving, an exception, and at its least forgiving, program termination. Notification-based systems may be the outlier here (still in heavy use), but even they are often frowned upon for being too 'loose' and unanalyzable, at odds with the ideals of static typing and static binding.