Ask HN: What makes a programming language fun?

I'd never really put much thought into it, but lately I've been thinking a lot about what it is that makes a language fun to use. I've put a lot of thought into what it is that makes a language useful, but not so much about what makes a language fun. So I figured this would be the best place to ask. So let me put it to you this way, what features would you want to have in a language that you would hack with in your spare time? What features would make you not want to hack with it in your spare time?