[DanRosenwasser]

As a PM on the team, I actually _share_ your concern! ;)

In fact, we're very mindful about the cognitive overhead of new features, but we're also motivated by pragmatism. These features have been highly demanded to help them write real-world code. My coworker Ryan has written up a pretty good explanation of why we tackled the feature set that we did: https://news.ycombinator.com/item?id=16277367

Keep in mind that TypeScript strives to model JavaScript as it's written broadly. While we could always take the stance that you have to rethink the way you write JavaScript, that would be unnecessarily stubborn. Hope that gives some insight!

[seanmcdirmid]

It is a bit dizzying that the type system keeps getting even more expressive (the next release might have type conditionals!) while the core language and its dynamic semantics is necessarily limited to tracking Javascript (so no operating overloading, no pattern matching).