I like the marking rule. It's an increase in melee stickiness rule.
If you melee a foe and attack him (you have to attack him), you can mark him, which means you get (i) adv on your OA and (ii) your OA doesn't cost your reaction - it does say you only get one one OA per turn (but each creature acts on it's own a separate turn). So, with multiple attacks, if you attack different enemies, you can get an OA against each of them if they move away, and your OA is at advantage. And you still have your reaction up your sleeve, too.
This is very different to the basic PHB OA rule, where you just have to stand next to the guy to get an OA if he moves away (you dont have to attack him). That OA is a normal attack however (not at adv), and it uses your reaction, so you can only get one of these per round. Marking provides a way for characters in melee to be much stickier, which i think is a desireable increase in complexity.