What people talk about category theory w/r/t Clojure? Since it has almost no type system to speak of, I'd be very curious to know what in the world they're on about.
Macros and the repl were, to me, the best part of clojure.
I see people talking about monads and applicatives all the time. I don't know Clojure, but I got the impression it has a powerful implicit type system.