So I thought I would post some of my positives while I’m at it:
Fantastic classes: Pretty much everything before PHB3 felt amazing (I just don’t have much experience with the PHB3 onward classes). They were all built to fulfill a clear fantasy as well as having a clear role. The idea of role was just brilliant, codifying and making effective things that were always there in DnD. Defenders could ACTUALLY protect others, the concept of Leaders finally allowing support classes to move beyond the healbot cleric, Rogues were allowed to be more than skill monkeys thanks to the Striker role, and narrowing the focus of the Wizard instead of letting them do everything was great for it and the other arcane classes. They showed the Swordmage could be its own thing beyond a Wizard with a sword, but be an actual fusion of swordplay and magic. I’m 100% convinced the Barbarian, Sorcerer and Warlock wouldn’t have been in 5e without the massive work done by 4e to give them life of their own. The way all the powers were so clear and unambiguous and that once you understood how to read powers and how roles worked there was basically no barrier of entry to any class. I liked that they had a primary attack stat and a variable secondary to add flavor to builds. Best Cleric I’ve ever played (even if the STR/CHA build was never supported), Warlords were cool, Fighters were powerful, Warden were inescapable, and I loved playing an Avenger. Mine was a Dwarf who fought shirtless! The Seeker was kind of a misstep but I think a primal range weapon controller had a lot of potential, but they should have played with the idea of trigger plant growth instead... the name was also weird 'The Seeker'?
Epic Destiny: Never made it to Epic but reading these was always a hoot. “Once per day, when you die…” is always a good time. Love the Dark Wanderer walking back from the underworld with no explanation

and going anywhere in the universe in 24 hrs is insane.
Weapons: 4e weapons were interesting. You had the give-and-take relationship between accuracy and damage, but also fun characteristics like Brutal or Defensive that could be played with. And I gushed enough about Implement that I don’t need to repeat it.
NADs: Making it so the offensive party is always the one rolling was just a great idea. It made attacking far more instinctive, and it made playing a support class WAAAAY better and satisfying. Granting a +2 to attacks to a spellcaster doesn’t feel like a waste when even Fireball is an attack roll. I tried to make a 5e Warlord and making it so they could support Spellcasters (and thus not be punished by party composition) proved nearly impossible without a ridiculous amount of extra text and twisting. Attacks in 4e are attacks, and a bonus is always the same value. It also gave a lot of nuance to certain characters. Rogues were allowed to target Reflex and Fighters had attacks that target Fortitude it was great!
Cosmology and the Primal Power source: I just love the cosmology in 4e, being focused on creating interesting places to explore instead of some weird Alignment essentialism. The mythology of the Dawn War, the complex relationship between gods where folks like Bane or Asmodeus, quintessential bad guys, were allied with the other gods against the Primordial, the idea of the Primal Spirits kicking them all out of the Prime Material plain, the origin of the Feywild and Shadowfell, the Underdark, and the interesting new gods… I particularly like Erathis, goddess of civilization, who is willing to do all sorts of morally dubious things to accomplish her goal. The Raven Queen, Torog the God who Crawls, the way Tharizdun is only mentioned in the DMG. The entirety of The Plane Above is just fascinating and full of cool concepts. Love it!
Combats: 4e hit when I was still playing Final Fantasy Tactics A2 and I ate up that tactical combat! You could make some great set pieces in the system. The environment mattered, the position mattered, and the game rewarded team work like no others. Party Optimization was more important than individual optimization and figuring out all the synergy between PCs was great.
Bloodied Condition: Just a brilliant little mechanical hook I wish was still with us.
Monsters: Just fantastic designs for a lot of them where as long as you try to use as much of their abilities as possible, their built-in strategy will emerge on their own without you having to think about it. Building encounters was easy and you didn’t have to flip to different pages to check what a spell did.
Healing Surge: Brilliant mechanic that gave PCs more HP for the day without turning them into bloated meat bags during combat. Their scaling nature and how they interacted with healing and how they became a daily pacing mechanic for the entire party regardless of class. Hit Dice are just a terrible replacement. And they were a great resource to use for out of combat. You could have harsh environment, or traps, or simply various rituals, take away Healing Surge. Second Wind on everybody was cool and gave another trigger to many different powers.
Rituals: Great way to not load up the Wizard with ALL the utility magic and not have them compete with combat powers. They were accessible with only a feat, allowing you to create characters who had a little more magic without needing to be spellcasters. Their material and ritual scrolls made for great treasures, and since they were generally costly they never felt like they could always trivialize someone’s skill and since everybody could pool their resource and multiple people could even help a ritual, they felt like a PARTY resource. It was no longer the Wizard just spending a spell slot to solve problems, it was the party contributing the material and sometimes healing surge, to make the Ritual happen.
Races: 4e Kobolds are just fun. Halfling’s reroll ability was a hoot. The Dwarf was my group’s favorite for their minor action Second Wind. Shardmind were a really cool concept. Wilders too. The way everybody had an Encounter Power was cool. Eladrin teleporting, the Rebuke of the Tieflings, the Dragonborn Breath… No boring races here!
I think I’ve gushed enough for now! God, I want to play some 4e again…