I was having trouble articulating what I felt was a true issue with 5e, and this post kind of answered it perfectly.
5e was designed to limit interesting choices.
Think about it. What choices do you make when leveling up your character? Ignoring for the moment feats and multiclass, since they are still labeled optional, and as such didn't receive the balancing time that they needed before publishing. You gain a level in the class you already are. You gain a prescribed number of hit points. Once in your career you choose a subclass. Five times in your career you raise a stat by +2 or two by +1. If you're a spellcaster you pick spells. Warlock has some invocation options, and maybe there are a few subclasses that pick maneuvers or something, but by and large, there are very few choices when building a character.
And even when there are choices, many of them are illusions. Reincarnate is strictly worse than Raise Dead. Eldritch Blast is better for a Warlock than any other damaging cantrip 99.999% of the time. Dancing Lights is a fun spell, but Light does 95% of the job and doesn't use Concentration. Material components are interesting, but just use a focus so you don't have to spend game time plucking rose petals. It doesn't matter what weapon you pick because they all only have one stat, which is what damage die you roll. It doesn't matter what armor you buy because you should just buy the one with the highest AC that you can afford.
But it doesn't end there either. Let's say the DM wants to set up a journey through a harsh wilderness to get to a fabled lost tomb. You need supplies. You need to hunt. You need to find your way. Nope, you have goodberry. You have a ranger who "can't become lost."
You're in a dungeon and a cloaker drops on you. You can't breathe! If you don't kill this thing in 21 rounds of combat, you might suffocate!
You're breaking into the royal treasury past hundreds of guards. Good thing there's a low level spell that breaks the core design principal Bounded Accuracy with respect to stealth, and nothing guards can do to boost their perception.
There's a certain expectation of "the DM can change the rules to suit the campaign," but this requires the DM to know all the rules. What I found as a veteran player but a new DM in 5e, was that I would design an interesting challenge, and it would be foiled by a shoddy rule or a simple non-resource. Then I can change the rule for the future, but the challenge is already gone. Why spend time on it? Even if the encounter "accomplished its goal" by nickel and diming the PCs towards running out of "resources" (read: spell slots), the design time took longer than the game time, and that spell slot will come right back when they rest. Or, they have so many spell slots that they won't miss that one anyway.
Players are in the same boat. You set up a tactically interesting ambush using fog cloud or darkness and then what? You don't get advantage, the enemy doesn't get disadvantage, everyone gets straight rolls. You try to maneuver and you get Opportunity Attacked. You try to grapple or shove and those things don't even do anything. So you just... attack. Your turn comes up and you attack. You wait and your turn comes up again and you attack. At 5th level, you get to attack twice, or your cantrip does two dice instead of one, but it's still not more interesting. You only get one action on your turn, and most of the time, the best way to reduce the enemy to 0 hp and save resources is to just attack.
Maybe if you're very lucky, you have a truly excellent DM, who foils the shoddy rules in advance, and comes up with interesting challenges, and rewrites the whole system to enable their truly excellent adventure. But then it's not 5e any more, is it? And you have to attract players who will accept that this spell doesn't work this way, and that hexblade is banned, and that we're using gritty realism rests sometimes, and that PC death is both likely and permanent, or whatever else is in your hundred page house rules document.
So, it's not that there's one rule that is The True Problem with 5e. It's not that they used the wrong font or that they didn't just reprint the AD&D DMG with a 5e sticker on it. The design philosophy of 5e is the true problem with 5e. It's a wading pool. You can get into 5e, and have a good time. You can stick your feet in while your kids learn, or have a beer with your friends, but you can't swim in it. You can get a bigger wading pool, but it never gets deeper. If you try really hard, you can immerse yourself in it, but then there's no room for anyone else.
