Entire systems are missing for no good reason, or are incredibly under-baked; skills, exploration, social encounters, magic item purchasing and crafting, any real use for treasure, and monster building I would all include here.
Feats and Multi-classing break the game in half. The gulf in power between a regular PC and one that has even taken 1 powerful feat or a 1 level dip into a frontloaded class is enormous. These variant rules do not function as intended because it seems they were never given a real balance pass.
Most turns in general devolve into move+attack/cantrip. Sometimes players have BA abilities, sometimes they don't.
The Monk in general is way too squishy and deals way too little damage. Out of all classes they struggle to participate with any other single-class peer.
Many spells, even low level ones, are way too powerful. Web for instance, as a 2nd level spell, creates difficult terrain on successful save, and on a failed save it restrains the enemy, a condition that only goes away when the creature saves after spending an action to get free. This wrecks the action economy of a group of enemies of any level that don't have some kind of special immunity. And don't get me started on Polymorph.
The suggested adventuring day of 6-8 combats and a few short rests has no enforcement mechanism. In some games I've played short rest casters or abilities get the shaft, in others they have way more short rests than they should and dominate. The rest and recovery mechanics are working on a broken assumption with no real incentive to follow it.
The game doesn't function at all into the mid to high levels. Enemies either bring immunities to conditions, legendary saves, or some of the more broken spells in the PHB or they die like pushovers. Rocket tag rears it's ugly head yet again. Breaking the game becomes a triviality to anyone paying any attention to their spell list or planning out their subclass abilities.
The "natural language" and "rulings not rules" standard of the PHB onward lead to confusingly worded and explained abilities, and the clarifications from JCs twitter were even worse to the point that the company forbade rules clarifications via twitter after a while.
And the ultimate nitpick: the text is left-aligned instead of left-justified, because apparently nobody at Wizards ever noticed or cared.
TL;DR Summary: the game is set up in a way that the people who will get the most value from the paid books are those that engage with the material in the least, leaving it to make up their own game on their own time or just ignoring mechanics entirely. It's been almost 9 years now and there's still a sense that the product is grossly incomplete. While some of these issues might be addressed in the new core books, in general I don't have a lot of faith on that front. Multiverse seemed to favor pushing things towards even more simple to the point that most caster monsters were severely nerfed. The Vecna stat-block was a joke, hardly worthy to challenge party members 10 levels bellow him. If Multiverse and things like it are an example of trends they are taking into the new core books I don't imagine it's a product I'll be too happy with.
