This is by far my least favorite change to the combat mechanics. I've tried it the way it's written and I just hate it. Specifically the encounter in the Starter Adventure at Wyvern Tor, with a half dozen orcs and an ogre.
Here's my main problem with this: Large creatures are terrible at opportunity attacks. Their reach expands as they get bigger, making a much larger zone of free movement. At the same time, if an ogre wants to be at it's preferred combat distance of 10', he has to take OAs from all the players within 5' of it to move back 1 square. I am kinda ok with the whole zone of control free movement around a target thing, but the minute you turn your back to go from 5' to 10' distance on a large target, that should trigger an OA, and it currently doesn't. It's pointless for ogres to try and harass ranged spellcasters or bow users because they can simply move back 5 and shoot without disadvantage because that rule specifically mentions 5' distance instead of "being threatened" or "inside enemy reach". That makes this OA rule extremely weak for large creatures.
Edit: I am the DM, clearly since I'm arguing for fair treatment of ogres. It's just terrible, to me, that being large makes you suck at zone control when it use to make you better at it.