immutable data structures, no mutable variables
first class function/high order functions
difficulties with setting up IDE and/or "first run" to get first results
macroses, understanding "compile time vs runtime"
Clojure model of handling multi-threading: STM/atoms
switch from imperative to functional paradigm. To start doing things in a very different way
ways of API design: data-driven, HOF, macros-heavy APIs
finding common used libraries, understanding Clojure dev stack