We can agree that the designers intended the feat system to be optional, but being optional does not exempt it from criticism.
I haven't seen anyone suggest that it does.
As I said in the passage you quoted directly above this paragraph: It doesn't matter whether you or I as individuals feel GWM and SS are imbalanced. The very fact that so many people do think they are imbalanced, and that the game is less fun as a result, is a real problem that can be fixed mechanically. Why would you not want to fix that?
If someone thinks something isn't doing what it is supposed to, they could be right, or they could be wrong. If they are right, there is something to fix, and if it can be fixed there isn't (normally) any reason not to fix it.
If they are wrong, however, there is nothing to fix. If I make screwdriver more hammer-like because someone complains that their screwdriver doesn't drive nails the way they think it should, I'm not fixing a broken screwdriver, I'm breaking it - and I might even manage to turn it into something that no longer drives screws in the process.
People thinking GWM and SS are imbalanced are, in my opinion, confusing a working screwdriver for a broken hammer.
If a player plays the game and finds it un-fun because they believe it is imbalanced, that is not the player's fault. It is the game's fault.
Not nearly as often as you might think. It all depends on
why the player believes it is imbalanced. In the case of the -5 to-hit/+10 damage trade, the folks I've seen believing that it is imbalanced cite reasons for that belief that indicate other things they are (or aren't) doing which if not done (or done, as appropriate) would remove all the reasons behind their belief.
Often, it is as simple as choosing to use more creatures with a particular range of AC, since there is a relatively easy to reach point where -5/+10 is equal or inferior damage to an unmodified attack.
First of all, my diction there was not great. "Always powerful" vs "situational" is not the right terms for what I was trying to say.
See, I was thinking from the perspective of a Fighter player. I don't have the empirical data in front of me, but it stands to reason that if you're playing a Fighter with a greatsword, you probably want to be good at fighting with a greatsword. Don't you agree?
I can agree not with what you have said, but with something near to it: If you want to play a character that is good at fighting with a greatsword, playing a Fighter and having a greatsword fulfill that desire.
The Fighter class is composed of roughly 100% combat features. There is mechanically no reason to play a Fighter unless your primary goal is to be good in combat.
You're "roughly 100%" is extremely roughly, considering that every sub-class of fighter has features that aren't combat features (remarkable athlete, student of war, know your enemy, some spell options, etc.) , and other things which all fighters share in common are not inherently limited to being combat features (ability score increases).
However, yes, the Fighter is designed so that every fighter is good in combat.
So when you see a feat that makes you even better in combat, it is very appealing.
Not necessarily. Being better at what I'm already really good at is less appealing than being better at something I'm not as good at yet.
By taking that feat, you gain "vertical" power in your specialist area. And that can mess up the game's balance, because now your Fighter with that feat is a better fighter than other Fighters without that feat. And that's bad. Are you with me so far?
Better at one-dimension of the character =/= better character overall, at least, not in a game like D&D where there is more than one-dimension of challlenges the character will go up against. And someone wanting to be the best at one thing really should be able to do it, which is how 5th edition works so far, and there is nothing bad about that.
Now, imagine your Fighter takes the Actor feat. You're not any better at your primary function (fighting), but you gain a "horizontal" ability that lets you do things you couldn't do before. Your Fighter is just as good a fighter as my Fighter who took Ritual Caster, but we can each do things the other can't. Which is also imbalanced, but in a good way, because now our characters are distinct and interesting without overshadowing each other.
I don't see the difference between a fighter with Actor and a fighter with ritual caster as being a different kind of difference than that between a fighter with one of those two feats and a fighter with great weapon master - one gets to be better at combat by way of doing things the other can't, but that other still gets the things they can do that the first can't,
and they have the advantage that said things which the first character can't do have likely opened an entire different style of challenge to be better handled by the character.
My idea to fix the optimization problem is to have feats be more "horizontal" and less "vertical." Martial Adept, for example, is a mostly-horizontal feat that could appeal to combat-focused characters without causing imbalance.
That you consider Martial Adept to be even partly "horizontal" as a feat for an already combat-oriented character does not make any sense at all.
As it stands, I am of the opinion that it isn't the feats that are problems. It is assumptions about other parts of the game, such as ability scores and their increases and how good a character that is "good enough" at something actually has to be, not matching to what the design of the game assumes the player will be assuming - and people point to the feats because they are more visible as being different from what prior versions of D&D established, where things like that a 14-16 in your "most important" ability score is plenty good and you will actually get more out of dedicating ability increases to getting more 14-16 scores than you will out of making this one score an 18-20 are not as apparent even though having that 18-20 is a large part of why a blamed feat performs as well as it does.