MGSV's gameplay gets a lot of praise, for good reason, but I do think it took a few steps backwards in becoming open world.
No swimming - this has been brought up before, every other MGS game has had some kind of swimming segment, or at least the ability to go underwater
Not able to press against any flat surface - this one is more than a small annoyance to me, in every other game you could push yourself up against any flat surface in the game, not so with MGSV
Snake's punch, punch, kick combo is no more - instead, the CQC button just launches into a scripted series of punches that always ends in a KO
No more rolling - yes, I know the dive replaced is the technical replacement, but it's just not the same
You really couldn't find a better use for the square button? - I mean come on, the dive is useful, but having an entire button dedicated to it?
Guards don't react to location based damage anymore - this one is also a pretty big bummer, guards still have hit animations based on where you shoot them, but they go back to animating as if nothing had happened afterwards, definitely a step back
Guard physics - this one may just be personal preference, but I don't really like the new physics in regard to how bodies act. I miss the mixture of scripted animation and subtle positioning physics form the previous games.
I think there are more, but this is all I could come up with off the top of my head.