People are talking about REPL development, and they are right, but one of the best things that you can do with CL that you can't do with other REPLs, such as Cl

People are talking about REPL development, and they are right, but one of the best things that you can do with CL that you can't do with other REPLs, such as Clojure, is... at least using Slime in Emacs, probably other editors too... when there's an error/exception or whatever, you drop right into the stack at the error location. You can see all the values of the variables. You can evaluate expressions. Travel up the stack and do the same things. That is SO powerful, and can save so much time debugging simple errors.