- Named arguments
- Multiple inheritance (traits)
- Incremental performance improvements over 1.9's VM
- Better compatibility with non-unix environments and small/constrained devices (embeddable)
- Sandboxed VM's (VM per thread)
Ruby 2.0 was (and hopefully still) earmarked to have traits which are an enhancement on mixins providing a much better & refined compositional implementation.
Traits are an improvement to mixins. IIRC Matz gave a talk (RubyConf 2010?) where he describes the proposed traits implementation. Here's one set of slides (not Matz's talk) which also shows
some traits examples - http://www.slideshare.net/fabiokung/ruby-20-to-infinity-and-...