There are a lot of great points above. I think type annotations are an excellent topic for a SPEC. It would be particularly valuable to have a conversation from the perspective of the scientific Python ecosystem. From the Rationale and Goals of PEP 484:
Of these goals, static analysis is the most important. This includes support for off-line type checkers such as mypy, as well as providing a standard notation that can be used by IDEs for code completion and refactoring.
A bit later in the document…
… third party packages would have to be developed to implement specific runtime type checking functionality, for example using decorators or metaclasses. Using type hints for performance optimizations is left as an exercise for the reader.
Though not the emphasis in the PEP, I suspect that the later would be a stronger driver for adoption amongst scientific Python packages.
Looking at the benefits and challenges from the perspective of scientific Python projects would be very valuable and (eventually) developing some sort of best-practices or procedure for consistent adoption across projects would definitely help push the pace for feature adoption.