What's that one concept that made you say wow in any programming or scripting language?

Goroutines.

KISS @ bash

Not much particularly. Although when i was learning java, i felt a newfound respect for python's super small code. For eg

x,y=2,3
x,y=y,x

And we are done . In 10 characters, we created 2 variables, initialised them and swapped them too. No classes, no main function Nothing. Super easy, super sweet

setjmp / longjmp in C.

GOTO keeps making me go wow, not in a good sense tho