The 8086 inherited the separate I/O space from the Datapoint 2200, a "programmable terminal". Since the Datapoint 2200 had a custom TTL processor, it was convenient to build in instructions to manipulate its I/O hardware directly. It doesn't make as much sense for a general-purpose microprocessor, which is why the 6502 etc didn't follow that approach. (Separate I/O instructions make sense in something like the IBM System/360 mainframes, where they were executed by a separate channel controller and gave you more performance.)

As far as the implementation of I/O in the 8086 chip, I haven't come across anything particularly interesting yet. It's essentially the same bus state machine as memory, except it signals an I/O access rather than a memory access.