A couple of things that come to mind:
Multithreading
Allowing the game to process more data such as simulating physics and other systems without slowing down the rendering or input threads, which allows for more smooth gameplay and more complex systems.
Networking
The network code in Unreal 4 is significantly better structured than Source's systems, with better implementations of popular systems such as linear interpolation and other prediction algorithms.
Rendering
Unreal Engine 4 has a very complex rendering system when you compare it to Source. For example, fluorescent jackets could easily be made in Unreal 4, where as this would be hard to do in Source. Also think about better lighting and shadows. Also way more detailed models.
Object Oriented
Lua in it's nature is not an object oriented language: it's procedural, which means that the code tacks onto eachother as the programmer writes code. This is different in C#: code can be called from nearly any point in the code, and an object (class, method, variable) can be instantiated and referenced nearly anywhere as long as the code for that object is present.
Also the support of static classes and the ability to start threads ourselves adds a whole lot more posibilities.
As C# is a managed language, a whole lot of libraries become available to developers, such as encryption and scripting. You could essentially implement a scripting language such as Lua into your own plugin!
