People confuse IOCP with Windows' Overlapped I/O. kqueue supports completion notifications; it all depends on the semantics of the event filter and its flags. Likewise for epoll. For example, both kqueue and epoll support pollable I/O completion notifications for AIO, which is the the analogous Unix API for Overlapped I/O. (Similarly, people conflate implementation details with architecture, such as when people explain that AIO isn't like Overlapped I/O by describing how AIO and Overlapped I/O is implemented, without explaining how the API necessarily makes it so.)
The benefit of IOCP and Overlapped I/O on Windows isn't the design. The benefit is that it comes complete out of the box, whereas on Linux and *BSD you either need to roll your own or supplement inconsistent kernel interfaces that people tend to avoid. But both IOCP and Overlapped I/O are higher-level APIs than traditional Unix readiness notification. The problem on Windows is that there's nothing like epoll or kqueue, which is critically important when you're trying to write library code that works with different event models. (Even on Windows Overlapped I/O isn't always ideal, especially in libraries trying to avoid callbacks or support multi-threading strategies different than those dictated by Overlapped I/O). Windows does implement something equivalent to traditional Unix readiness notification internally--it's how IOCP and Overlapped I/O are implemented--but its unpublished and exceptionally opaque. See https://github.com/piscisaureus/wepoll
The benefit of IOCP and Overlapped I/O on Windows isn't the design. The benefit is that it comes complete out of the box, whereas on Linux and *BSD you either need to roll your own or supplement inconsistent kernel interfaces that people tend to avoid. But both IOCP and Overlapped I/O are higher-level APIs than traditional Unix readiness notification. The problem on Windows is that there's nothing like epoll or kqueue, which is critically important when you're trying to write library code that works with different event models. (Even on Windows Overlapped I/O isn't always ideal, especially in libraries trying to avoid callbacks or support multi-threading strategies different than those dictated by Overlapped I/O). Windows does implement something equivalent to traditional Unix readiness notification internally--it's how IOCP and Overlapped I/O are implemented--but its unpublished and exceptionally opaque. See https://github.com/piscisaureus/wepoll