Ask HN: What language features have particularly appealed to you?

What language features have you come across that particularly appealed to you?

This isn't a "which is your favourite language?" question but rather which feature of a language most appealed to you. I was thinking the other day of which features of various languages most appealed to me and came up with the following list:

* Erlang's Pattern Matching * Lisps Macros

* The ADTs in ML Variants

* Go's Interfaces

* Clojures Protocols

* Tail Recursion in various Functional languages.

What would make your list?