I think my biggest problem with these feats is simply the fact that the feel like "must have" feats. I understand that nobody is forcing me to take them, and that by taking them I am giving up some other feat, but really no feat comes close to providing as much capability in combat as these feats do. They are the single highest contribution to DPR you can get from an ASI.
With how limited ASIs are to begin with, I was actually hoping that feats wouldn't significantly affect DPR, but would primarily focus on new options and capabilities. Shield Master is a good example of such a feat. It allows you to shove as a bonus action, a potent ability, but not one with a pure DPR boost. For players that want a pure DPR boost, they can increase their primary ability score. Since it is capped at 20 though, there is an upper limit to your combat potential. Once capped, players can choose feats for unique abilities or RP enhancing feats.
But with GWM and SS, all this changes. Suddenly there is one more feat you can take to increase DPR (on top of the other amazing benefits the feats give). Suddenly, The max potential damage is pushed skyward yet again. Suddenly you lose a lot of incentive to choose flavorful or interesting feats in the name of pure raw combat power. Sure, you can choose not to take them, but then you are missing out on a 15-20% boost to your DPR (25-40% for barbarians). This boost increases even more if you have a party with bless or a reliable source of advantage (wolf totem barbs, familiars using help, spellcasters using CC, etc). The boost is almost too large to pass up. It is far more of a boost than a mere +2 to an ability score gives (weren't feats and ability scores supposed to be "balanced").
What is worse, is that those feats marginalize other combat styles. The duelist, the dual wielder, the martial artist, etc all can't keep up with GWM and SS. Without those feats, they are at least in a similar ballpark, still lower mind you, but not overwhelmingly so.
To me it seems obvious where the problem lies. Change those feats and you will see much wider variety of characters. No longer will every two handed warrior take GWM and every archer take SS. No longer will dual wielders, rogues, and monks feel incapable of dealing a relevant amount damage after level 11. No longer will DMs have their encounters mopped up in a single round. No longer will the player who choose to be simple and just take ability boosts be completely and utterly outshone by the one with feats. No longer will crossbow expert and polearm master combine to give the fighter or barbarian a whopping +50% boost to their DPR. No longer will DMs have to artificially inflate the challenge of encounters, resulting in little more than an endless treadmill for the players. To me, that all sounds good.