Title says it all, what are some interesting concepts, features, etc. in programming? From anything to popular languages like Haskell and C, to less popular or experimental/research languages, what are some interesting features? Examples of use, etc would be great too! :)
While there are certainly many interesting "features" in the broad sea of programming languages, a risk of enumerating them is that of missing the forest for the trees. More concretely, the things which make certain PLs really, truly great tend to be <i>gestalt</i> "features" that arise from well-designed (or just lucky) combinations of traditional features. For example, both Ruby and Python could be checklisted into appearing <i>very</i> similar on a feature basis. But I'll argue that DSLs are more easily designed and implemented in Ruby, stemming from the details and combinations of its features. Related, Haskell is reknowned for being "DSL friendly", but due to completely different interactions of its features as compared to Ruby.