Ranger is the only class that has an anomalous design, which is why it's not surprising that the class was subject to redesigns already. The fact that almost all the class's offensive power (save hunter's mark) comes from the subclass abilities, and that really limits the scope of future designs. Personally, I'm not at all a fan of pet classes, either, and I'm sure that's part of my problem with it. The class is super narrow because it's wedged between Fighter, Barbarian, Rogue, Bard, and Druid, and that leaves a very, very small design space with which to work. Also, I still don't understand why 1/3 casters get cantrips, but 1/2 casters do not.
Beyond that, it's not really overall class design that's a problem. Just little bits.
Fighter's Indomitable is genuinely the worst ability I've found in 5e in terms of power level and fun, and it's the only ability you get at two different levels.
Berzerker's Exhaustion mechanic is a bit too punitive to be usable very often, making the subclass much less valuable than the Totem warrior.
Valor and Lore Bard are a bit uneven in comparison to each other, but that's mainly because Valor increases diversity and Lore increases focus.
Druids get far too much durability out of Wild Shape. This is a general problem with the Polymorph rules still being pretty broken, however. 4th level spells significantly boosting damage and granting hundreds of temp hp with basically zero downside is just silly.
Life Cleric is much better than the other domains. Light is the only domain that really keeps up. The War cleric should be getting Extra Attack at 8th level; if a Bard can do it, so can a cleric. The clerical spell list is really boring at high level, however.
Paladins being able to Smite multiple times a turn allows a nova ability that's pretty excessive, especially with abilities that allow extra attacks. I've seen other tables limit the ability to once a round.
Five Elements Monk is terrible, as it heavily overloads the Ki pool and the spells are basically cast like you're a 1/2 caster so you're super behind. Wall of stone at level 17 is not impressive.
Sorcerer... Honestly I've never seen anybody play one for more than a session. The number of spells known is seriously restricting, and the spell list just amplifies that fact. Metamagic is a very interesting concept, but the rest of the spell list isn't well executed. Nobody has bothered because it doesn't look very good on paper once you start planning it out.
Warlock does not work well with multiclass rules. Devil's Sight is a silly ability. The "on hit" nature of several invocations and hex combined with the Eldritch Blast cantrip is also very silly. It makes the class do an extraordinary amount of damage, and having played one for a one-shot it feels genuinely like cheating. Beyond that, Pact Magic feels fairly weak due to the short rest nature of it. 2 spell slots until level 11 is dumb. All the Invocations that take spell slots and are limited to 1/day are horribly bad. Pact of the Blade feels very weak due to the loss of Eldritch Blast. Not that you can't do good things with a Bladelock, but man do you have to put effort into the class to do it, and really just love the flavor of the class overall.
Wizard has some uneven subclasses. Abjurer and Diviner are amazing. Illusionist and Transmuter are great. Conjurers are ok, but I don't like pet classes. Enchanters are weak because the school is so sparse and the spells very brittle. Evokers are weak because they're so narrow, and silly because the 6th and 10th level abilities can basically never work because of the spells that exist. Necromancers are weak because they focus on one of the few objectively evil actions in the game, and I hate pet classes.
Beyond classes:
I don't like classes that rely heavily on short rests. Simply put, I dislike the short rest mechanic. Warlock, Fighter, and Monk all suffer in the games that I've played in because the majority of the party is almost never interested in a short rest after about level 4-5. We might get one a day, but that's it. It's long rests all the way.
I don't like the -5/+10 feats, nor Crossbow Expert and Polearm Master which grant bonus action attacks to stack with the -5/+10 action. Feats in general are poorly balanced, even if they are essentially necessary for late game advancement for Fighters.
I don't like Bless. It's an absurdly powerful spell at all levels of play. I'd rather it were just the old version: a flat +1 bonus to hit and fear saves for 6 rounds to all allies in 20'(?) without concentration. Yeah, that might be always good, but it's closer to 1st level in power. And you could bring back prayer at 3rd.
Stoneskin is so terrible. At the very least it should resist all damage, mundane and magical. It's a 5th level spell, and most of the dangerous opponents you face at 9th level just outright ignore it. Also, it doesn't need to last a flipping hour.
Six saving throws is dumb. Yes, it's "elegant," but it's really dumb. There, I said it. I completely buy the need for adding Str saves as well as having Dex and Con saves. I do not buy the need for Int, Wis, and Cha saves. Both Int and Cha saves feel very arbitrary and, IMX as both a player and a DM, feel like cheating when you ask for them. I say pick 1 mental save, and keep the 3 physical saves. Then move the Initiative bonus from Dex to Int. Cha doesn't need any more help.
Dex outpacing Str so much is pretty lame, as is how generally useless Int is overall. RAW all being smart gets you is minor resistance to Feeblemind and Maze, while Dex is just better than Str unless you go GWM.
The CR of creatures at high level really doesn't stand up at all. Yes, we get it: Feats, multiclassing, and magic items make the PCs more powerful than their levels suggest. That doesn't mean that the only things that should be challenging to PCs after level 15 at all are Dragons and Demon Lords. There should be more monsters available to smack down even PCs using all the optional rules.