It has heavy reliance on pattern matching. In fact, `=` isn't even technically assignment, it's the match operator. Assignment is more of a consequence of matching (though it doesn't have to happen, eg: `1 = 1`). All that to say, most Elixir codebases are written with types in mind, and many are written with pattern matching that would cause a type error at runtime. The new type system just builds off that and moves these errors to compile time (well, not JUST that but ya, this is just meant to be a quick answer).

One important thing that is often not mentioned is the lack of operator overloading. In Elixir if you have "a + b" it means "a" and "b" must be numbers for the code to succeed, which narrows down the possibilities significantly. Compare that to Python, where "a + b" applies to numbers, string concatenation, and any object that implements the __add__ or __radd__ magic methods, it becomes a nightmare to type.