It's hard, because 'Fixed' is even more subjective then 'problem' (which is already situational). I don't really have a prepared list, but if I had to run through it as I can think of what I've seen from the discourse around it, it'd be something like this:
Subjective Stuff:
- The Rules Glossary. Almost everyone seems to agree this was universally an improvement.
- The Book. The books neater looking and far prettier. Lots of fancy art.
- Weapon Masteries. This is highly controversial territory, and I personally view them as more in the problem then the fix column, but they are undoubtable food for a starving martial population.
- Exhaustion. While most people liked the UA Exhaustion better than the printed version, most people like either version better than the 5e 2014 Exhaustion.
- Starting ASIs. People probably prefer Background to Race as the origin of the ASIs, but most people preferred them unlocked instead of either, and this is a very divided issue at best.
- More Stuff. Everyone gets a lot more stuff. Origin feats, more class features stacked on top, more flexibility to swap stuff. D&D 2024 characters are a good bit more powerful than 5e 2014 characters. This is definitely not an unalloyed good, but some people will prefer it.
- Healing Buffed. Healing, especially at low levels, was buffed quite a bit. Bonus action healing potions, cure wounds/healing word/etc all got +1 die of healing.
- Feat Selection. Powerful feats got brought more in line with other feats - Lucky, GWM, SS, etc. Weaker feats got buffed (Dual Wielder, Shield Master, etc). There is still winners and losers here though.
Mechanics:
- Weapon Drawing. If they hadn't borked it immediately be implementing the incentive to do Weapon Swapping, I actually like the change the draw weapons before attacking, since it makes a lot of thrown weapons work better without the need for a Fighting Style or Feat. This is tarnished by also being part one of the bigger problems though.
- Better Monks. Most people seem to think the new monk is better. The bug around Stunned creatures being able to move, most people think a stronger Monk with reworked stunning strike is better.
- Conjure Spells. Conjure Animals (and all Conjure Spells) were probably more broken in 5e 2014. The new ones are mixed, but the old ones were very problematic.
- Nerfed Spells. The spell fixes were very underwhelming, but a lot of changes were technically improvements. Forcecage is still a problem, but any nerf is better than no nerf. Leomund's Tiny Hut sees situational nerf. Animate Objects was nerfed somewhat. Weird was buffed to not that bad.
- Baked In Updates. Stuff like improved Dragonborn are baked into the default.
- Divine Smite. Divine Smite was nerfed pretty hard. Mileage may vary on how necessary this was.
- Sorcerer Spell Lists. Sorcerers have subclass spell lists (mostly). This is a fairly popular change.
- Warlock Improvements. Warlocks now know their subclass spells instead of just expand their list. Most people seem to prefer this change. Pact of the Blade got some of the Hexblade features by default, which most people like.