My two main grudges against Typescript:
1) Union types can't be passed as arguments if there is a variant for every element of the union
2) No tuple polymorphism, i.e. [T, U] isn't assignable to [T]. This is not a mistake because the length of the arrays differs and therefore they may be interpreted in a different way, but IMO there should be a tuple type which is actually an array but length is unavailable and it supports polymorphism. This sounds stupid, but since function parameter lists work well with tuples it would actually enable a lot of functional tricks that are currently inaccessible.