Not quite true. In earlier versions (pre-3E), facing affected whether or not a character received their shield bonus to AC. And in 1E, a rogue could backstab if attacking from directly behind his target.
There have been a number of attempts to mimic real world melee combat, with varying degrees of success. Personally, I hope 5E has a fairly rules-lite base, with most of the mechanics mentioned in this thread presented as options. Nothing wrong with one group using marks and auras, another using attacks of opportunity, another ignoring this altogether.
AHHHHHH FACING
Sorry, too many "I knocked over the mini now what" moments with that concept

Anyway, I'd hope that they at least do an Aura Defender (the essentials Knight, for instance) in the core build. That's streamlined, rules light, and works well (maybe too well... super-duper knight indeed).