Web is not a medium. Web is a space of possible combinations of medium, message, and method. The medium and the message are familiar, but what exactly is a method? Methods are algorithms and effects. Previous media had no algorithmic component, no way to conditionalize or effect outcome (choose-your-own-adventure books came closest).
Web is not interactive. It is a two-way audience: every viewer can be a publisher. But the websites themselves are not publishers, they are the medium. Every website is a new medium.