A lot.

It's much faster. Learn more at http://www.rubyraptor.org/ and http://www.rubyraptor.org/how-we-made-raptor-up-to-4x-faster...

It has better tooling for helping you to identify and solve problems.

They've blogged about their enhancements here: http://www.rubyraptor.org/how-we-made-raptor-up-to-4x-faster... and Part II: http://www.rubyraptor.org/pointer-tagging-linked-string-hash...