One thing I like to mention, in Python at least, is that a decorator is syntactic sugar for an assignment statement. (I'm a bit more reticent to use the term "syntactic sugar" in the case of Hy).

You could mention something like: 

> The two following expressions are equivalent:
> ``` clj
> (with-decorator inc-decorator
>   (defn foo )
> ```
> and
> ``` clj
> (defn foo )
> (defn foo (inc-decorator foo))
> ```

I think this can be improved further to be made clearer, but I like to show that it's just an assignment.

What do you think?