Standardizing casters is one thing. I don't like it, but I can appreciate the virtues of it (especially if there are still some outliers). WotC's poor presentation of the subtle differences between different spellcasting classes in 5e sewed a lot of confusion for new players. The better solution would be sidebars highlighting differences and perhaps hiring some writers with a background in education, but just eliminating the differences is also a solution to something that I believe was actually a problem.
BUT standardizing them to prepare spells directly proportional to their spell slots at each level is ungodly stupid. To what, save a column on the damned progression charts? Someone at WotC just needs to deeply reflect on what they're even doing. Now casters have to suck more at level 1, get an unneeded power boost through most the game, and, at the height of the power, will have an ultimately very disappointing final few levels because they prepare so little variety in very high level spells. Casters are just more quadratic, while ultimately less satisfying.
And there is, of course, the three spell list system. As presented so far this just gives an overwhelming amount of choice to new players (or players new to a particular list), and makes choosing Bard spells (which requires finding the spells of certain schools among the Arcane list) without online tools, a guide, or system mastery, a herculean task compared to just reading the Bard list.
I voted dealbreaker. I don't know if it will actually be a dealbreaker, even if it makes it to the finished product, because that goes to the eventual strengths of the new edition overall. I don't know if there is some redeeming quality I will eventually find in this whole, vast, seemingly aimless reworking of 5e that actually sells me on it. So far though, it just seems like a bunch of random changes to ensure that everyone has to buy new books to play at a 5.5 table. There are a few changes I like, but none that seem vital. Basically the only thing I wanted out of a new edition was seeing the late 5e tweaks in Tasha's and such integrated into the PHB for convenience sake, but there is a limit to how much stuff that I dislike I'll put up with for the sake of a more concise book. The playtests still have a few more chances to wow me (not as many as there will be playtest documents, because I'm certainly not going to read them all if they don't start giving me something I like), but so far I am generally unimpressed, and if I had to pick one thing to be a dealbreaker it would be the new approach to spellcasting. It's not the thing I hate the most (even after they've seemingly walked back the alternate crit rules, it offends me that it even briefly existed), but so far it's the thing I hate that will be too deeply ingrained in the system to houserule around.
I might have been sold on radically reworking a bunch of rules to rules I didn't like as much if it was for a consistent greater purpose I cared about like accessibility. The saving grace of many compromised and flawed 5e designs is that they served accessibilty in some way. But I don't see that sort of overriding purpose so far. If the purpose of standardizing spellcasting, level progression, etc. is simplicity, I worry that WotC has confused structural simplicity and accessibility (as many commenters do) and they are NOT the same thing at all, or even as closely related as they might seem. Yes it is conceptually similar on a grand, archetectural scale for the system to only have three spell lists, and for people trying to achieve absolute memorize-the-lists system mastery it simplifies things, but that sort of simplicity doesn't make it more accessible to new, casual, or really just non-obsessive players, instead it just expands what needs to be mastered to roll up a character.