I know that there's arguments for and against classes and functions. In recent years, I've come to love functions. But they are both tools that have cases of appropriateness.
However, I think a lot of people are looking at classes the wrong way.
In this thread, I'm hearing that classes should encapsulate functionality. While this is an aspect of classes, I don't think it's the main use case of a class.
A class should represent a type of state. That is, if you create an instance of a state, it should only support the attributes and default properties that are defined for it. This is mainly for shorthand(there's usually a syntax to build instances of classes) and for clear documentation. At a lower level, the availability of a class construct makes it possible for AST parsing to do interesting things to your OO code, although I don't know how common that is outside of JavaScript. If JavaScript didn't support class syntax, it'd probably be more difficult to implement things like decorators, both in terms of the Babel plugins that support it as well as future native syntax support. But I digress.
The way I see it, a class should be responsible for holding a mostly fixed set of attributes, setting defaults for those attributes, and have properties that compute off other properties.
What doesn't belong in a class are methods that do complex things with other objects and class instances. If your method does something complicated that affects outside state, it's time to extract it to a function that takes a context argument. This makes it easy to test your function without necessarily having to meet all the requirements of the classes you'd be using with it, and it makes the code more flexible because you can use it for other contexts besides a single class. (I know that you can use inheritance and composition to kind of do the same thing, but I think this sucks because inheritance introduces problems and composition just adds more code than just importing a function where you actually need to use it.)
More concisely, a class shouldn't be looked at as a module, or a set of related functions that do complex things, especially when not all of those functions rely on a state. That's the job of an actual module construct in the language. Classes have a specific job, which is to build an instance of a type. If your class has a lot of functionality that's not totally dependent on the class instance itself, it doesn't belong there in my opinion. Just because a class can be used like a module doesn't mean that it should.