Knockout.js is a JavaScript library designed to simplify dynamic user interfaces by using the Model-View-ViewModel (MVVM) pattern. 

Here are the key features that differentiate Knockout.js from other front-end frameworks:

1. Declarative Bindings: Knockout.js uses a declarative binding syntax, allowing developers to connect the UI to the data model easily. This helps in keeping the HTML clean and intuitive, specifying bindings directly in the HTML elements.
2. Automatic UI Refresh: One of the standout features is its automatic UI update mechanism. When the underlying data model changes, the UI automatically updates to reflect these changes without needing explicit DOM manipulation.
3. Observable and Computed Observables: Knockout.js introduces observables and computed observables. Observables are special JavaScript objects that notify subscribers about changes. Computed observables are functions that are automatically re-evaluated when their dependencies change, providing a reactive approach to UI updates.
4. Dependency Tracking: The framework has built-in dependency tracking, meaning that it automatically tracks which observables a computed observable depends on, ensuring the correct re-evaluation when dependencies change.
5. Templating: Knockout.js supports templating out-of-the-box. This allows developers to create sophisticated, dynamic UIs by defining reusable HTML structures that can be bound to different parts of the data model.
6. Compatibility and Lightweight: It is lightweight and can be integrated with other libraries and frameworks. Knockout.js is not as opinionated as some other frameworks, which makes it versatile and easy to incorporate into existing projects.
7. Extensibility: The library is highly extensible, allowing developers to create custom bindings, extend observables, and integrate with other tools and libraries.
8. Ease of Learning: Knockout.js has a gentle learning curve, making it accessible for developers who are new to JavaScript frameworks. Its simplicity and clear documentation contribute to its ease of use.

These features make Knockout.js a powerful tool for building dynamic and responsive web applications, especially where a clear separation between the view and the data model is desired.