As someone who loves Io, it gets my attention because:

— the language is very mutable, almost all behaviours can be modified

— it's simple, all you can do is send messages to objects

— you can write DSLs and macro-like things without special macro support

— it's so dynamic you'll end up live-coding without realizing it

It has less syntax than Smalltalk, it's homoiconic like Lisp, but doesn't need support for macros because of the way messages are handled.

A not intolerable way of thinking of Io is it is to object calculus what Lisp is to lambda calculus. There are parts where that analogy breaks down but it's not a bad first approximation.

edit: formatting