I was thinking of loosely categorizing complaints I see about 5e based on what a person's actual favorite edition was
CR doesn't work, monsters are boring (4e): At this point we can say that there isn't much of a point to using CR in 5e except as a very rough guideline. Combat is very hard to balance in 5e and increasingly difficult at higher levels: either it's a cakewalk or a tpk and/or the DM has to fudge rolls and adjust things on the fly. I think this group of players is attracted to the "combat as sport" tactical aspect of the game, and DMs want to authentically play the opposition in a way that 'automatically' provides a genuine challenge but is not unfair.
Not enough character options, no magic item economy (3e): 5e has intentionally sought to release very few sourcebooks, and moreover the sourcebooks only reference the core rules, not each other. These players seemed to like depth and customization of both character options but also specific kinds of subsystems like crafting.
Both of the above categories of players are annoyed by "rulings not rules." They want more options and support, and a robust, balanced system where both players and DM can at least mildly optimize encounters without anything "breaking."
The lore of 5e should be more internally consistent and consistent with other editions. There's also not enough of it: we need detailed settings and supplement books that reference each other (2e): 5e has taken a light touch when it comes to lore. Not only have they said that 5e is explicitly not consistent with lore in other editions, but they have released very few classic settings and have even focused most of their FR material on the sword coast. Due to popular demand, they've started to change that, but even there they keep a light touch; they are seemingly aiming to evoke the feel of the earlier setting but not the copious detail, and they make some changes that core fans of those settings have complained about. A subset of this group is upset about the way inherent evil is depicted in the game (via alignment, monster lore, etc).
5e is in fact too ornate and complicated. It should be even simpler and able to be presented in one core rule book. Instead of rules, the game should provide procedures for dungeon-, hex-, city-, point-, and other -crawls (Basic/Rules Cyclopedia/OSR). I'm not sure Basic dnd was anyone's favorite edition prior to the OSR. In any case, this group is all about "rulings not rules," except that 5e provides too many rules and too much exception based design to make this a reality.
Gygax was a wargamer, and his style of play was competitive (between dm and player, and between groups of players at open tables). Also Critical Role sucks (1e). Ok, that last part is a bit unfair. But I think the trick here is to look at the 1e DMG as an obtuse key that
unlocks the secrets of the game. These players are very committed to the sandbox aspect of the game as well as an Appendix-N setting base, and thus see the adventure path format and heroic fantasy as an anathema, from the Dragonlance adventures onward. They vocally dislike the aesthetic and ethos of "original character" style play, e.g. extensive backstories, hanging out in character doing non-game related things, the idea of narrative arcs.
Ok so in this game you roll a d20 and try to roll high, and the dm tells you what happens. Seems like a perfectly sensible game. What's the problem? (OD&D). Playing dnd is about throwing out 95% of the rules, and you can do that as easily in 5e as with any other edition.
Thus concludes my TED talk.