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?
Because the syntax is powerful, concise and readable at the same time. It closely reassembles mathematical set notation, making it fairly easy to grasp. Moreover many interesting problems can be expressed in terms of list comprehensions (like the Norvig's spelling corrector[1]).
[1] http://norvig.com/spell-correct.html