When I started using Swift the first thing I started missing was the support for abstract classes. I was used to using abstract classes for my ViewModels, especially to implement the template pattern, but also to provide some basic methods, like showing dialogs, that the ViewModels may need. Of course there are some “tricks” to do abstract classes like checking in the class type in constructor and throwing an error if it is the abstract class type, but Swift is a language based on protocols so there are better ways to achieve the same results just using the protocols.
Imagine you want to add a functionality to show iOS alerts to some of your ViewControllers. In a language like C# you would create an abstract class, something like
BaseViewController, add a
ShowAlert method to it and make all your ViewControllers inherit from this base class. Most languages nowadays only support single inheritance, so you would put all the functionality your ViewController may or may not need to your one abstract class. But you can typically implement as many interfaces as you like.
Using Swift and protocols there is another way. Protocols in Swift are really similar to interfaces in languages like Java or C# but with some neat stuff added to them like default protocol implementation.