[jgw]

Programmable syntax (reader macros), programmable printer are 2 big ones. Also regular macros are key, manipulating code as structures not text, having both lexical and dynamic variables, closures and all the other nuts and bolts stuff. Type and compiler-optimization declarations help keeping things fast (well, not fast enough, but I'm trying!)

[jwdunne]

Thank you :)

I think you were correct about Clojure. I read recently reader macros are not an available feature.

[arohner]

Yes, mostly. reader macros are not extensible by the user. The compiler has and uses reader macros.

Rich Hickey has said he hasn't seen an implementation of reader macros that is compose-able across libraries. i.e. two different OSS libs using a two different incompatible macros.