What's New in Ruby 2.0.0?

Ruby 2.0 is here (sort of, a preview release is here)!  What's new?  What do you need to know about?

Put simply, there are a lot of new things, but there are no major architectural changes.  There are a number of very useful new features such as refinements and keyword arguments.  But this looks to be a much smoother release than the 1.9.x release, which took time to gain acceptance.  Ruby 2.0 is even available on Heroku right off the bat (even before the official release), so it should be safe to start using very soon.