A few omissions that jumped out just from my quick scan:
- String.index & String.rindex
- Array.index & Array.rindex
Would you be interested in a language that lacked these classes?
I also think they should work with the RubySpec (http://rubyspec.org) folks to make sure the executable spec is in sync with the written one.
But the Ruby core unfortunately isn't known for really reaching out across the Pacific, mostly because of the language barrier. :-/
Some might argue that we should use Ruby 1.9 as the primary reference, but Ruby 1.9 is moving fast and its features change frequently. Once we draft the specification based on Ruby 1.9, the specification and Ruby 1.9 would get quickly out of sync.