I'm not a rubyist but I'm really excited for the russian-doll caching and really hope it catches on in other frameworks. Data dependency tracking makes everything from straight-up caching to real-time updates effortless and I think is one of the few places where an ORM can be really helpful (queue the two hard problems quote about cache-invalidation and naming things). Quora has been doing this since the beginning and I'm surprised it hasn't caught on in other open source (namely python) frameworks to the same degree, hopefully RoR will add some fuel to the fire.
If I remember correctly, Adrian Holovaty said something about exploring pjax solutions for Django when he returned to help manage the project. It would be neat to hear more about his ideas if I missed them posted somewhere...
