Adv from flanking is a particular problem because adv doesn't stack. Getting it from flanking makes a whole list of tactical options and just cool abilities almost entirely redundant.
It also makes Elven Accuracy kinda busted.
I like the movement in 5e. If I were to change anything, it would be having OA's trigger when a creature leaves a threatened square, not just when leaving reach. I do not like the aesthetics of moving freely around an enemy, but the general freedom of movement is a massive improvement over 3e.