All true Lisps have a very important feature: you can enter any expression any time. (Subject to some static rules of course. E.g. you can't use a GO outside of an enclosing TAGBODY.) Heck, you can redefine a class while you're in the debugger that's waiting for you to continue with a restart.
So, I was quite shocked to learn that Python, heralded as a dynamic language, doesn't allow you to redefine classes. Of course, redefining classes isn't something you do all the time, but in Lisp implementations, supporting stuff like this is simply a question of honor.
Thank god Slava is kicking everybody's asses left and right outside of Lisp proper. I hope this will lead other dynamic language implementors to adopt the same ethos.
In my own Lisp implementation adventures, I was (of course!) seduced to think that maybe such total dynamicity could be sacrificed. After all, I can just restart the app I wrote to have the changes take effect. But in the end, it just wouldn't be a true Lisp. And it would add a load of bollocks to the language and its specification.