The point of Ruby's class library is not that it is feature-filled. It is that it resembles Perl, and shares with Perl the ability to make it easy to whip up answers. Any decent Smalltalk has a library that has more features than Ruby. But said library is unlikely to shine in scripts of length 10-50 lines. Ruby behaves similarly to Perl in this space.

You can get a sense of this by taking a look in the book at the chapter on built-in classes and methods.

Moving on, I do not believe that Ruby's mixins differ significantly from what you would use with CLOS. I could say that more definitively if I knew CLOS better. As for the blocks, well the block/iterator concept for Ruby is stolen straight from CLU. If you know it there, then you know it in Ruby, right down to using yield to call your associated block as a callback.

So I don't think there is that many new ideas in Ruby. Just a mix of ideas from different languages in one framework, bound together very well.