@Jack Rusher It would have to open a second dimension I guess. It isn't really a debugging tool, but it's a mechanism that supports debugging tools (and more). As such it is among the best I have seen, even better than its Smalltalk equivalent (a more standard exception system). But the overall debugging experience in Common Lisp is less pleasant, partly because of the information overload created by compiler verbosity (at least in SBCL, the only Common Lisp I have used seriously), and partly because of the large distance between the source code and the debugged compiled code for macro-intensive code.
One of my long-term but low-priority to-dos is to check out the demo versions of commercial Common Lisp systems to explore their debugging experience.