Most importantly, the finished MIR revolution [0] will finally allow developers to focus on new exciting features, like ergonomic "impl Trait" [1], incremental compilation, or even the beginnings of Gc [2].
I was surprised to hear this and immediately thought "but why would you do such a thing!?" but I read the blog post and I'm impressed with the forethought that I certainly lacked. This is freaking genius.
[0]: https://github.com/rust-lang/rust/pull/35764
[1]: https://github.com/rust-lang/rust/issues/34511
[2]: http://manishearth.github.io/blog/2016/08/18/gc-support-in-r...