I haven't played much 5e, I was comparing it more to 3.5, I am pulling from loose memory and the spotty options that exist online (due to the game system license which honestly didn't help 4e's case), but if this source is to be trusted for the things that have been made creative commons, it shows the charm person power. It's an encounter power with the standardized to hit mechanics the effect is:
The target gains vulnerability to charm and illusion effects from you and your allies, and suffers a -5 penalty to their Will and Sense defenses until they snap out of it.
Compaired to 3.5's from here:
This charm makes a humanoid creature regard you as its trusted friend and ally (treat the target’s attitude as friendly). If the creature is currently being threatened or attacked by you or your allies, however, it receives a +5 bonus on its saving throw.
The spell does not enable you to control the charmed person as if it were an automaton, but it perceives your words and actions in the most favorable way. You can try to give the subject orders, but you must win an opposed Charisma check to convince it to do anything it wouldn’t ordinarily do. (Retries are not allowed.) An affected creature never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing. Any act by you or your apparent allies that threatens the charmed person breaks the spell. You must speak the person’s language to communicate your commands, or else be good at pantomiming.
This is obviously only one example, and a particularity egregious one at that, but speaks to the sorts of differences I saw in what 4e was trying to do, and what 3.5 was doing. I don't think it's fair to attribute the dissatisfaction specifically to grognards, when these are very clearly different kinds of systems, and the goals of 4e are much more video game like in having controlled variances and results, rather than the freer form of 'rulings not rules' intended games.
If that's not your style than, rock on, but I feel like the counter culture revival of 4e does gloss over the fact that is really was a pretty drastic shift in what DnD was, and it was disliked authentically for the very different and opinionated choices it made, not just online backlash.
That's valid, we might have under utilized rituals in replacing much of what I felt was lost in vancian casting. I still feel the homogenization of powers, while very sensible from a mechanical standpoint, stood out to me as very video game.
I can see you're point in spell slots use for environmental vs combat, I think that was part of what I found interesting in caster classes in 3.5, and later pf1.
I get that there is a lot of intelligent design in 4e, and I think on a mechanical level it makes a ton of sense, but I think ultimately it comes down to rules vs rulings mentality to the game. I would say it was very much on the side of rules, and for many players that felt much more like the MMOs they knew than a TTRPG.