A few notes on Sticky Cover: most modern warfare title's combat take place at ranges vastly inside the actual engagement ranges of those conflicts.
So while that may be a sop to playability, suggesting it's particularly realistic is incorrect. Particularly if there's also surfeit of Chest High Walls.
Second, any installation or target worthy of protection and capable of landscaping won't have Chest High Walls every 5-15ft for the comfort and safety of attackers.
Well, maybe if they were just painted cardboard and a few had fragmentation mines hidden in them...but probably not, and definitely not if you're being serious. There will instead be elevated shooting positions that give defenders both a superior angle of fire and protection from retaliation.
And possibly more, but that's the minimum.
Lastly, the Refresh action always had a flaw that, IMExp, made it nearly unusable: to benefit from it you to not be attacked during the round of recovery. Not even hit, just menaced. I think this was a smart, plausible detail -trying to get your drek together under fire is famously difficult, but it still rendered the effect mostly useless. Even if you had Total Cover and you couldn't even be hit, even a little blind Suppressive Fire ate the AD.
Random association, also Cheap Shots seemed under-useful. They were on that short list of things that never got used.
As for what I like...?
To start: weapon balance and qualities (no obvious or totally superior standouts), variable error and crit range, stress damage, bags full of guns (and sacks of hammers), damage saves (the math was annoying, but the results were worth it), Aiming and Bracing, Basic Feats that encourage team-work, the Gatling quality, Chase Scenes, how Flash and Bang damage worked -really I could probably do your query better by inverting it and you can quite literally assume that what I don't complain about is Just Fine.