We've been mixing and matching playtest rules with 2014 rules for about a year now. It's fairly seamless. However, encounter balance is off more than it used to be, mostly because weapons-focused classes all hit MUCH harder, and they were already topping the damage meters. I'm assuming the MM changes that WotC has alluded to, which are designed to make creatures live up to their CR, are going to help fix this. For now, you just have to prepare your encounters to be a bit stiffer.
In terms of game play, 2024 is mostly quality of life stuff, and part of that is making classes viable in more situations. For me, using rage to improve abilities like perception seems weird, but our barbarian players really enjoys the increased utility, making his character less of a one note brute.