Lots of critics saying "why not just use an existing language"? One thing people seem to be missing about TS is that it already has a BETTER type system than most previous mainstream high-level languages.
It's structural instead of nominal. In C# and Java you can have an interface IFoo with method Bar, and a class C that has method Bar with exactly the right signature, but C doesn't implement IFoo unless you edit the definition of C accordingly.
Whereas in TS, C already implements IFoo merely because it has the right structure.
This is already an excellent improvement, but when they add generics to TS it's going to be a big improvement on C#. An interface could be used to specify a constraint on a generic parameter, but it doesn't mean that a generic argument has to nominally implement the interface. It just has to have the right structure.