• Union types

I know this is something folks have been clamoring for for a long time. It looks to me, though, like you're still dealing with object casts, under the hood, so not sure how useful this will be.

• Closed hierarchies

Neat. Niche, but neat. I could see myself using this.

• Labeled break and continue

Boy, this one feels like it should have existed a decade ago. I will ABSOLUTELY be using this.

• Extension indexers

I.E. we can now polyfill Range support. Cool.

• Collection expression arguments

I am absolutely mindboggled by anyone who thinks this syntax is a good idea. Seriously, it's EXACTLY as verbose as the thing it's trying to replace. What problem does this solve?

• Memory safety

Sounds neat, I guess. Doubt I'll ever use it.