When learning a new programming language you sometimes come across a language feature which makes you wish you had it in your other programming languages that you know.
What are some language feature which were at the time of learning very new to you and that you wish your other programming languages had.
An example of this is generators in Python or C#. Other examples may include list comprehensions in Python, template in C++ or LINQ in .NET or lazy evaluation in Haskell.
What other semi-unique language features have you come across which were completely new and enlightening to you? Are there other features of older programming languages which were unique and have fallen out of fashion?