Live Programming and Images
I've been dabbling with Lisp and Pharo, and the most exciting concept to me was live programming. You don't have to write source code, compile it, and then run the program. The two acts have become one. The code you've just written can be used right away in your program that is already running. Variables values, function, and class definitions can be modified immediately, and your program will work with the new specification.
Both Common Lisp and Pharo use the idea of images, which is not only your software, but a complete development environment that lets you make that software. They also allow you to modify said environment to better suit both the program and the act of programming the software. The synergy result in a much tighter feedback between the act of reasoning (writing source code) and the tangible result (computation).