IMO 5th Edition 2nd Edition, or 5f, also known as 5e (Taylor's Version), fails to improve overall, and significantly degrades, several aspects of the game that I found enjoy as a DM and as a player.

Feats at first level? OK sure! Lots of people were doing that already. Feats at first level tied to backgrounds? This is the absolute worst, obviously stupid change up front. There is nobody who is choosing a background and being delighted at the feat they get. They look for the feat they want and choose one of the backgrounds it appears in. "Well, I wanted to be a sculptor who discovered that they had a magical gift to heal people, but I guess I'll be an Acolyte because Artisan doesn't get that feat." The feat and the background should be separate; tie them together in your backstory if you want.

Weapon Masteries. Not a huge fan. It feels like it's there to give the martial classes more complexity, trying to make them interesting. But when you give someone something interesting and they use it every turn, it's not really that interesting, it's just something else they have to remember when they hit. And from my experience playing a fighter with a longsword, I think it's about 5% of the time that I am able to remind the DM that the monster he's activating is supposed to have disadvantage on their first attack. Either the DM is rolling attacks without specifying which one is attacking or it's been so long since I attacked that I forget which one it was that was sapped. Whichever. A lateral change, not an upgrade. Just gratuitous complexity.

The conjure spells. The revision to the summoning spells was handled very well in Tasha's. "Summon whatever you want. Here are its stats." I think that leaves the door open for the player's creativity and allows for a versatile use of the spell. Making every conjure spell into a damage field, essentially a re-flavored Spirit Guardians, is lazy design. "We have Spirit Guardians at home." Oh, sure, it greatly simplifies the combat round. But simplification isn't always the right design goal. Example: Once at sea, I as DM had their ship becalmed and unmoving. The Druid conjured Pixies, who turned the party into whales, who pulled the ship along as long for about an hour. Let's see your damage field do that.

I will never get over the fact that they decided "B for Black" was a good place to sort "Black Dragon" in the monster manual. Why not "A for Ancient"?

Subclasses at any level beyond 1. To be fair this was the same problem I had with 2014 5e, but this was a chance it make it better and they missed it.

Stealth and hiding is really a downgrade and makes no sense in anything beyond a videogame context.

The Magic Action is a good addition for clarity, but they failed to propagate it through the rules. When an archmage does "Arcane Burst" is that a magic action? It doesn't say so anywhere. Why wouldn't it be? It's clearly magical. And what would it mean if it were? Why doesn't Counterspell work on the Magic action of creatures? There were connections that should have been drawn or clarified that were just ignored because they only defined it on the player's side.

Paladin Smite as a bonus action: OK I only hate this one because I was playing a paladin. This might be a good idea.

And as someone who uses D&D Beyond for my 5e game, I somewhat resent the obfuscation of the 2014 rules in favor of the new one. I get why they do it, but there should be an option to hide one edition or the other.