I was just going to post about this. In Graham's early essay on succinct languages, he asks, "Are there languages that force you to write code in a way that is crabbed and incomprehensible?"
Factor feels incomprehensible until you learn to hold the stack in your head. The result is you eliminate all the intermediate variables in a program, which makes the language extremely succinct. All that's left are function calls and control-flow, which you can't remove anyway.
I believe the key to succinctness is (1) minimal boilerplate, (2) higher-order functions and (3) reducing intermediate variables. Functional languages achieve (3) with function composition. But Factor (and Forth et al) goes way beyond what functional languages can do.
