The one that sticks out for me is condition handling, particularly restarts. I'm someone who reads books about programming more than I actually program, so I don't know how actually practical Lisp's ability to do extremely sophisticated responses to error conditions and the like (including choosing where to throw to programmatically at the error site) is. It strikes me that it might be one of those things -- like hot-reloading -- where there are so many factors that it's mostly impossible for humans to get right. But!! I've never seen it in any other language, and I think about it a lot.

@phoe-krk's book on it, https://news.ycombinator.com/item?id=24867548 is very good.