Rich absolutely argues that basic data structures like hashes are simple -- something that is generally agreed upon. They are simpler than 'objects' because you can use basic comparison operators on them, and you can operate on them with higher-order functions etc.

The question which parent is digging at is whether a larger, complex app, that leans heavily on hashes actually results in an app that is on the whole simpler, i.e. does 'a simple thing plus a simple thing equal a simple thing'. This is something I've heard discussed well on the Ruby Rogues podcast -- I think that either David Brady or Josh Susser may have a good blog post on the subject of 'simple + simple != simple' but I'm struggling to track it down.

Whilst I love Rich Hickey's talks I do find myself coming round to the same conclusion as parent -- if you forgive my possibly incorrect interpretation of their argument -- that the idea that simple data structures are simpler is fairly useless if programmers use that fact naively.

PS. Everyone should watch the linked talk, it's brilliant, and one of my favourite programming talks ever. I recommend it to every programmer I meet.